I kept asking this kind of question on GW\'s forums (unti they closed them down)...
I think the \'appropriately sized gaming base\' thing is there to stop people basing a single miniature on a massive and incredibly OTT scenic base that hugely detracts from the mini itself.
The thing to remember is that the single mini category is meant to be about the painting (and to a lesser extent conversion/modelling) of a single miniature and not about building a complex base for it. That said, if you look back at winners from the last few years a good few of them have had pretty OTT bases (like entire stained glass windows behind them).
What mini is it that you\'re planning on putting on the 60mm base?
I\'d say it depends if it looks oversized or not.
by example, a Tau Battlesuit comes on a 40mm round base, but the Forgeworld resin ones come with 60mm bases, and they look fine on 60mm bases.
A terminator captain or similar though, which comes on a 40mm round base, would look silly on a 60mm base.
One option which people have used to effect is to put the mini on a standard base, mount that base on a larger display base and put some of the scenic elements on the display base so the mini itself is removeable as a gaming figure.
IMHO
Ben