I looked at Alamay and Photoshelter and both suggested in thier FAQ that upsizing was allowed. I think the rule is that it doesnt degrade the image though.
http://www.alamy.com/contributors/st...al-cameras.asp
Unfortunately i cant give a link to photoshelter as it was in one of the sign up sheets and i dont have my log in with me atm (i signed up but I've not uploaded anything yet).