we come in all sizes

[Steelblood]

The high end of medium and the low end of large, how do you know when you swap sizes? Is it height, mass, weight? Is the dwarf the only race that can count as more than one size when it's beneficial?

[AslanCross]

The relevant table is here under "Big And Little Creatures in Combat."

Goliaths are Medium and stand from 7-8 feet tall. They seem slightly larger than Bugbears, which are about 7 feet. Goliaths have Powerful Build, so they count as large for when it is beneficial.

Skullcrusher Ogres (MM3) are about 8 feet tall and are Large.

Since when were dwarves counted as more than one size? They're Medium.

[Yora]

For humanoids, it's usually measured in body height.
2 ft and taller is small, 4 ft and taller is medium, 8 ft and taller is large, 16 ft and taller is huge, 32 ft and taller is gargantuan, and 64 ft and taller would be colossal.
For quadrupeds, make it body length and set body height at about half that value.

Or another rule of thumb: If a humanoid can ride it, it's one size category larger.

[Steelblood]

The relevant table is here under "Big And Little Creatures in Combat."

Goliaths are Medium and stand from 7-8 feet tall. They seem slightly larger than Bugbears, which are about 7 feet. Goliaths have Powerful Build, so they count as large for when it is beneficial.

Skullcrusher Ogres (MM3) are about 8 feet tall and are Large.

Since when were dwarves counted as more than one size? They're Medium.

I could have sworn in the 3.5 PHB it notes that dwarves can count as small when it helps them.

[Devils_Advocate]

In D&D 3.5th Edition, dwarves aren't treated that way at all, though several races, e.g. half-giants and I believe goliaths, are. I've seen that suggested as a modification to dwarves, though I find it awfully odd that someone familiar with 3.5 would misremember that as RAW, and also odd that someone unfamiliar with it would be asking about size categories. Unless your group plays with that version of dwarves.

According to the relevant table, everything but height/length can vary from what's listed, so height/length would be the number that determines size category, if you want to have size category determined by some number. (If you don't, then you can just select whatever seems most appropriate.) Which makes sense, since it's the only quantity given that directly measures, y'know, size in any sense.

[AslanCross]

I think the Arctic Dwarf from Races of Faerun is small-sized, but it doesn't have the in-between category. The Midgard Dwarf in Frostburn is a Small-sized outsider.

I think there is a Slight Build trait, but I'm not sure what has it.

[kamikasei]

I think there is a Slight Build trait, but I'm not sure what has it.

As far as I know, it's only appeared for Kobolds in either Races of the Dragon or the web enhancement thereof.

[magic9mushroom]

As far as I know, it's only appeared for Kobolds in either Races of the Dragon or the web enhancement thereof.

Web enhancement.

[Steelblood]

It’s been a while since I played 3.5, and not having the book on hand makes it hard to check things. Maybe I’m just losing my mind :p

EDIT:yup i'm going crazy.

[KillianHawkeye]

Allow me to direct you here.