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Monster Manual (page 337) describes the Spider as a Tiny beast. In terms of 5e, "Tiny" means any size from 2 ft. and lower (correct me if I'm wrong). As a DM, how should I describe the size of the Spider? Would it be a realistic big spider like black widow or tarantula, or a fantastic overgrown spider, or a tiny bug?

It has 5 ft. Reach and 20 ft. walk speed, so my first guess was a big fantasy spider 2 ft. wide.

But then I've found this question, and the answer portrays the p. 337 Spider as a domestic house spider that could slip under doors.

On the other hand, this question assumes that Tiny creatures are ~1 ft. wide, which is larger that most of the known arachnids (it turns out there is at least one).

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    \$\begingroup\$ The first question could do with answers that don't specifically address imps and sprites. Nonetheless, it is still open and could be answered to address what is being asked here. UrhoKarila's answer is on to something in his/her first statement, I think. \$\endgroup\$ – daze413 Nov 16 '16 at 3:00
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You're pretty much correct: a spider in D&D has an upper limit of around 2 feet in size, with no particular lower limit. I'd also note that they have only one hit point, so are presumably pretty small.

In particular, the chart Size Categories on PHB 191 says Tiny creatures can control in combat a square roughly 2 1/2 feet on a side. Since humans control a 5 foot square, I'd say it's reasonable that a spider would have at most a leg span of 2 feet.

In general, though, the entry for Spiders is pretty simplistic. Google tells me there are over 40,000 kinds of spider in the world, and this one entry covers all of them. Deciding just how big or small a particular spider is falls under DM purview.

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  • \$\begingroup\$ But size matters, doesn't it. What if a Druid says "I Wild Shape into a Spider"? Would it slip under a door? \$\endgroup\$ – enkryptor Nov 16 '16 at 5:29
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    \$\begingroup\$ I'd let the druid himself decide whether he becomes a 1 cm or 1 foot long spider - to be stated when he uses Wild Shape. \$\endgroup\$ – timje Nov 16 '16 at 10:11
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Sizes in the Dungeons & Dragons scale with the 5' side area. Taking a size factor F, this can be put into a formula: \$F*5'=L_{side}\$ and \$A=L_{side}^2\$. This makes a square occupy 25 square feet.

Factor 1

Medium and small creatures are a factor 1, they have a side length of 5' and occupy a full square.

Factor 0.5

A tiny creature is half of that for 2.5' side length. This means that 4 fit into a 5'square. Typical Examples are cats for the mid range, but we should check one factor lower to see how small we can get.

Factor 0.25

If you get smaller, you leave where D&D5 has terms for. In earlier edition you would have got a "Diminuitive" creature, occupying a square with 1.25' side length. 16 fit into the 5' square, occupying 1.5625 square feet. This is by far not RAW, but can help to determine the size.

Interpretation

We know that beings are always a bit smaller than the square they occupy - the main difference between small and medium being height. But how much space do you need to effectively control an area of 5' by 5'?

First of all, you need to effectively reach from the center of your occupied area to the one next to you, because you need that to attack a creature in the neighboring square. So, in best case it is once the creatures \$L_{side}\$, in the worst case this demands us to have \$L_{side}\times\sqrt{2} = L_{Arm}\$.

Virtuvian Man

Now, time to pull out the virtuvian man for a medium creature: \$2\times L_{Arm}+L_{Sholderwidth}=L_{Height}\$ and \$4 \times L_{Shoulderwidth}=\frac34\times(2\times L_{Arm})=L_{Height}\$

Shifting the numbers around this gives us a size of \$L_{Side}\$ for the average size and an absolute maximum height of \$L_{Height}=\frac32\sqrt2 L_{Side}=2.12\times L_{Side}\$

So, for the average we get 5' and as absolute upper limit of a medium creature, we gain 10.24 feet.

Virtuvian Spider?

Now, assuming this 'Virtuvian Man' can be applied equally to smaller creatures, especially the Tiny Category, we get \$L_{Side}\$ as the average height of such a being, \$\sqrt2 L_{Side}\$ for maximum. Well, for a spider, we speak about the absolute length with stretched out legs. Assuming it is more or less just a scaled up Black Widdow, then each of the 3 parts of a leg is as long as the torso, for a total of 5 to 7 roughly equall length pieces (2*Leg+Torso=5*Torso) forming the length.

Conclusion

So, the spider is like \$2.5'\text{ to }2.5\sqrt2=3.5'\$ in diameter, for a torso size of \$6''\text{ to }8.5''\$ with legs of at least 1 foot long. Which is a scary big spider, considering the largest spider we know today Theraphosa blondi has a diameter of 1 foot and would perfectly well fit into the size limits of a tiny creature.

Since there are no smaller sizes, this is the very upper limit of our spider's size - it could very well be much smaller!

With previous generations in mind:

If taking diminuitive size into account, the smallest tiny spider would have a diameter of 15" to 21.2", being larger than the Bird Eating Spider.

Diminuitive spiders would range between 7" and those 21", filling in exactly the gap of that Spider Beast,

Anything smaller than 7" would have ben a "fine spider". Not fine as good but as D&D 3.5 size category.

But again, as D&D5e stopped with that kind of below tiny categorization (like fitting 64 tiny spiders into a 25 square feet area before forcing you to apply the swarm template for more creeping horror): Your \$Tiny\text{ }Spider^{TM}\$ only has an upper limit of about 3.5 feet fully stretched with no real limit to the underside.

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  • \$\begingroup\$ That basically means that the Spider, being a Tiny beast, can NOT slip under a common door nor into a keyhole. \$\endgroup\$ – enkryptor Nov 16 '16 at 10:39
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    \$\begingroup\$ @enkryptor Clarified the conclusion: That is just the very upper limit, while lower limits only applied in previous generations: your tiny spider could have a (fully stretched out) diameter of 0.1" as a Bedbug or 1' for the Tarantula or even 3.5' for that GIANT FREAKING RADSPIDER before crossing into the next size category. \$\endgroup\$ – Trish Nov 16 '16 at 11:11
  • \$\begingroup\$ What about that particular Spider from page 337 of the Monster Manual? \$\endgroup\$ – enkryptor Nov 16 '16 at 11:13
  • \$\begingroup\$ As small as you like them to be, though on my table I would prefer to beef my Theraphosa blondi to 2 HP and the "Radspider" to 3 - it wouldn't change the game anyway, unless you have a group of gnomes with 1d4-1 slings, then 3HP suddenly change from a 1-hit to a 2-hit monster. \$\endgroup\$ – Trish Nov 16 '16 at 11:23
  • \$\begingroup\$ Size matters. Check the linked question - rpg.stackexchange.com/questions/57001 \$\endgroup\$ – enkryptor Nov 16 '16 at 12:21

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