In this blog post, Vlad Zamfir describes a bet as a object with a signature.
What kind of signature will Casper bets have? What is the length (in bytes) of those signatures?
We're planning on abstracting the validator's credential verification policy, actually, to be programmable, rather than forcing everyone to use EC crypto, for example.
The size of the signatures, however, will be bounded above. By what? Who knows, at this point :)