I'm of the opinion that it's fair to ask for the correct base size, but not be unreasonable. E.G. old terminators on small bases is acceptable, and not a huge advantage anyway...makes plasma cannons and demolisher cannons that much more effective. Similarly, larger bases on 'showcase' type models, such as seargents, champions, HQ units, etc. is acceptable and falls under the rule about not penalizing a model for being "awesome." However, there are limits, and more than 1 size up or down is significant and wrong unless there is a good reason. E.g. if a GD-competitor wanted to use his duel model in a game and it was his chaos lord facing down a SM captain on a MC base, would you really have a problem with it?