RAW, the reach of an unarmed strike should be 5 feet unless otherwise noted.
From the 5e Player's Handbook, page 195:
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
Emphasis mine. Unless otherwise noted, a 5-foot reach is the only guideline we have. RAW, then, monsters have a 5-foot reach unless they are explicitly noted to have a different reach.
What does it mean for a monster to be unarmed?
Obviously a monster carrying a polearm and trying to attack with it is making an armed strike, and in the case of a weapon with the Reach property that can have an explicit reach of more than 5 feet.
But that won't typically change a monster's reach for an unarmed strike-- they can't attach a fist or a foot to the end of the polearm and then punch or kick from 10 feet away as an "unarmed strike".
There is an interesting possible exception when we come to attacks with natural weapons, like tentacles. These are attacks defined in stat blocks and aren't listed as unarmed strikes, so it's a plausible ruling that such creatures have an unarmed strike available in addition to their natural weapons, with a default range of 5 feet.
But it's also plausible to decide that a normal body part isn't the same thing as a distinct object wielded as a weapon, and so attacking with one is, for all intents and purposes, an unarmed strike, and would have the range described in the stat block entry for that attack.
I think each DM would have to adjudicate that for their own table, but it might be a pretty rare case. For monsters with natural weapons, attacks with those weapons are generally going to be better than a default, available-to-all-creatures unarmed strike would be.
But it's so big!
Here we leave the happy realm of RAW and start interrogating and extrapolating from what's published.
If it seems counterintuitive to you that a larger creature should only have a 5-foot reach, it seemed that way to me as well, but that is nonetheless my conclusion. My reasoning is as follows:
RAW doesn't give much wiggle room. There are no other rules written on this that I'm aware of, so a RAW standard leaves us with what actually was written whether it seems realistic and sensible or not. RAI and houserules can be different but are inherently not RAW.
Reach is calculated from roughly the center of a creature (with some leeway for obscure body plans). Unusual structures can be expected to be addressed by text in the stat block on an attack.
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.
Which is a longwinded windup for: the most direct interpretation of the 15-foot reach for the Greatclub attack is that the Stone Giant is wielding a club that is between 7.5 feet and 10 feet long or so, roughly similar to the proportion we might expect from a medium creature wielding a "normal"-sized club. If it's not using the Greatclub, it has no way of covering the extra distance described in the Greatclub attack.