If my character, a medium-sized creature, were to drink a potion of growth, would his reach with a weapon increase?
The rules as written don't address this situation all that well. I argue that your character's fundamental reach would increase, but mechanically this would still be represented by the 5-foot area surrounding the spaces your character occupies. See my answer here for more details on why I think that would be so, but the relevant portion of my reasoning is:
A medium creature takes up one 5'x5' square (1x1 squares on a grid), its body is centered in the middle of one square, and can reach up to 5 feet out in any direction from any edge of that square.
A large creature takes up more squares (10'x10', which is 2x2 on a grid) and so its body is centered on a vertex rather than the center of a square. From that point, it (unarmed) already covers 5 feet to get to the edge of the squares it's occupying and then extends an additional 5 feet into its engagement range. This pattern scales with bigger size classes.
It may also, as a result of its height, have effective vertical distance to deal with in its reach (a 10-foot tall giant can't swing its arm out straight in front of it and expect to hit a 5-foot tall enemy), but that's just flavor and is not represented explicitly in the rules.
None of this has any bearing on a weapon such a creature might pick up:
- A Stone Giant's Greatclub attack has a reach of 15'
- Stone Giants are Huge (15'x15' space occupancy, 3x3 squares on a grid), and about 20' or so tall
- 3x3 squares places its center such that it is 1.5 squares away from the outermost limit of the space it takes up overall, roughly 7.5 feet from its origin or "shoulder" to the edge of the space it occupies
- A Stone Giant, unarmed, can reach (stretching, lunging, what have you) an additional 5 feet beyond that without movement
- So its fist (for example) can lash out a total of about 12.5 feet from its center, which is 5 feet beyond the limit of where the giant, itself, is on the grid
- This is farther, in absolute distance, than a medium creature could extend its arm, and so it does have a "total reach" longer than that of a medium creature
- But because it has to cross proportionally more squares than does a medium creature to extend that arm beyond the space the creature itself is occupying, that additional reach beyond its own size (as expressed on the grid) is not as dramatic as one might assume
The standard frame of reference for this is corrupted somewhat, as the game does not define space smaller than 5 feet very well and so may overstate the reach a medium creature has. It's honestly weirder that a medium-sized creature can freely reach a distance roughly as long as its own height without moving than for a giant's reach to be described as I did above.
Extra weapon reach beyond that can be gained with a weapon that explicitly offers more reach, just as for a smaller character. In keeping with the "spells do what they say they do" ethos, enlarging weapons alone does not increase their reach, and so whether or not you buy my arguments about reach changing with enlargement (despite not threatening farther-away squares than before) a character will not be able to reach further than 5' beyond its own square(s), with or without a suddenly enlarged weapon, simply from being subjected to enlargement by any means.
No; Being Large doesn't inherently increase your reach (from the edge of your space); you just have a larger space to reach from
To quote V2Blast's answer to the question "Does the Primeval Guardian ranger's Guardian Soul feature increase your reach twice?":
In general, creature size is not directly related to reach. It allows you to control a larger portion of the battlefield simply by being larger (and thus blocking a wider area, being able to be adjacent to a greater number of enemies, etc.). Some Large-size (or larger) monsters may have a greater reach, but this does not define a general rule that applies to all creatures of a larger size.
This is supported by the answers to this question: Does enlarging a character with a reach weapon affect his threat range?
Enlarge Person: Does the attack reach of the target increases with it's size?
only if the spell specifies it does - there's no inherent reach by size IIRC
In short: being Large doesn't automatically affect your reach.
Nothing in the rules states that large creatures automatically have increased reach, and so they do not.
And to quote DaleM's answer to the question "Does enlarging a character with a reach weapon affect his threat range?":
Spells do only what they say they do - Enlarge increases the size by 1 category and increases the size of the weapon (doing more damage) but does not increase the reach of the weapon.
Remember that a creature enlarged from medium to large effectively increases reach by 5 feet (more or less) anyway since it now occupies 10 feet of space rather than 5.