You could drive around without a front plate, or a smaller plate, but if you get pulled you could get fined. If you've got your plate in the boot and can attach it there and then you might get away with it.
There's a specific letter height and amount of spacing between each character that it needs to be. It's not the size of the plate, it's the size of the characters and spacing between and around them.
If you go for an MOT without you're full-size plate they will fail you for it, I always keep mine in the boot just in case.