I think 6"x6" - the globes we used for our mickey lamp post where half the size of the big one and it looks good. From the looks of your picture though, it looks like the 'a' in the middle would be a good size to put for the ears - how far across is it?
Yes, that's what I thought you meant, I think 1/2 the size of the @ symbol itself should look good - I would cut circles out of paper first, hold them up, then decide. I had meant that I thought that the a inside the @ looked like a good size - Your ears would be inside the oval, somewhat partially above the T and B, right?
BTW, I love reading your posts - you give great tent camping advice!