Hmm im not a convention artist, so i wouldnt know too much about sizing and the like, but i do know that commisions are very very big at cons. A lot of people like to have artists draw them, and prices very based on quality (so a little chibi drawing would be 1-3$, an anime drawing of their face for 5$, and full body/full color/extra characters would range from 5-15$, depending on different variables)
so if you plan to do any commissions, which are generally pretty popular, make sure yoy have like an iphone so you can take refference pictures of people, google picture refferences of characters, and contact people when their art is done. That last part is optional though, as some artists just give a time range like "come back in twenty minutes", but its watever you feel better with ^-^