I'm wondering if anyone can tell me what size of cake I would need. It's going to be a square and is for a birthday but there is only myself and my partner in the house and will only really be for a few people - maybe 7 or 8 maximum but obviously I can give a chunk away to people. I was thinking about 10-12 inches square, would that be too big?

That's too big. A 4" square serves 8 (industry standard 8 cu. in. servings) 1x2x4 inch slices.
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

A 5" square serves 12.5.

A 6 or an 8 inch is what i would do. But surely there will be leftovers. And also you can reduce the height of the cake to 3" so that you will have less cake.

Thanks for the replies. I think I need to get my tape measure out to see exactly how big it will be as I'm just guessing. I think it is a bit big though.

I'm hoping to put either a topper or a printed topper on top - would a smaller cake still be able to hold this?

Goodness, my sizing is really awful!!

As for putting a 'topper' on a small sq it definately depends on the topper. Without knowing what this topper looks like no one can say if it will fit.

It will hopefully be a 3d speedway bike with a rider, if that isn't feasible (struggling to find someone who is willing to try and make it) then it will either be a 2d version on the cake or just a printed picture.