The issue of basing is still a little fluffy unfortunately. The guidelines for the Youngbloods cattegory say:
Your entry should consist of any single human-sized Citadel miniature either Warhammer, Warhammer 40,000, Mordheim, Necromunda, Blood Bowl or The Lord of The Rings, mounted on an appropriately sized gaming base.
So what is appropriately sized? Very unclear!
There was some debate last year over base sizes in certain categories. We have therefore chosen to use the words \"appropriate sized gaming base\" in several descriptions. The intention here is to allow a little flexibility whilst keeping to the spirit of the competition. So, if you want to mount your Terminator on a 40mm base because it \"looks nicer\", you can still enter it in the Warhammer 40,000 single model category. Similarly if you prefer to base your fantasy model on a round base instead of a square one you will not be penalised. However, please note the judges reserve the right to move models that are not on appropriate bases into a category they deem more fitting.
At the end of the day though if you enter the best painted, most characterful mini, a Warhammer model for example, but stick it on a round 40mm base you are still going to win. Last year\'s winner was exactly that, a night goblin on a 40mm 40k base.
You\'d be pushing it if you went over 40mm in size, but up to that you should be fine.