The one which came with my car was a 155x80 on a standard Lotus steel rim. The wheels on the car are the 185/70 & 175/70 options, and although I've never used it I think it's roughly the same diameter. That fits in the front with no problem. I just flip the lid over centre so it holds itself open and there's plenty of room.
I'm currently thinking about moving to 15" wheels (for the modern rubber selection rather than looks) and that might mean a new spare. I'm told folks buy the space saver spares for the MGF which has the same PCD (95mm = 3.75") and although being a 15" wheel, supposedly fits in the front. But before rushing out to buy one I shall stand the 155x80x13" wheel alongside the 15" wheels and see just how far out it is - I suspect it's close enough to be ok to limp home on.
Although the space saver idea sounds good, you still have the problem of where to put the punctured wheel afterwards. With that in mind I always carry a pack of 2 of those aerosol "instant fix" things, which are becoming the non-optional spare on many cars these days. My plan has always been to use the aerosols first, if they don't work then it's dirty hands time.....