Size modifiers?

[Vintrastorm]

Are there any size modifiers hidden somewhere? Reading the races the only size modifiers that seems to exist are written in the races. Lower speed and no heavy weapons.

The spell enlarge/reduce gives advantage/disadvantage and increased/decreased damage. It seems, however, to be totally an effect of the spell and not dependant on size.

How about effects from being X size categories larger/smaller? How does it affect grappling, for example?

[Rilak]

Being two sizes bigger or smaller allows you to walk through an enemy's space as if it was difficult terrain. Being friendly just removes the size restriction.
So a halfling can move through a shieldwall made of Ogres, taking only attacks of opportunity :)

[Triclinium]

The grappler feat mentions something about larger creatures, but I'd have to check as to the actual effect it has.
Other than that I can't think of anything regarding sizes.

[rlc]

I actually asked customer service about the grappler feat and they basically said thanks for letting them know about that. So chances are they're going to be taking the larger size thing out of the feat, because you can already grapple them. Or maybe they'll add a size to it.

[hymer]

I can't speak for hidden rules, but so far there are no explicit size modifiers as such. Big things tend to have higher strength scores and lower dexterity, small things have the opposite. But this is already part of their stat block, and there is no rule that need be remembered for size modifiers. It's always spelled out in the given ability, spell, feat, etc.