Is there a standard hole diameter for corner braces?
There are no standard sizes. 

Braces of the type you are considering most commonly have holes that are less than 1/4" and more than 1/8", somewhere around 3/16".  They are usually intended for #6, #8 or #10 screws. You could probably use 1/8" bolts, or maybe 3/16" depending on the brace. 1/4" bolts probably won't fit unless you ream out the holes.

Which bolts will be sufficient depends on how much strain will be on them and how many braces you are using.