If the sim card is too big, you can buy a sim card cutter to cut it down .... or if the sim card is too small, you can buy an adapter to make it bigger.
If you take a new contract out, with your existing provider, you can't keep your number - you can only keep your number if you swap networks. I know it sounds daft, but that's how it was, the last I heard.
If you swap providers, you get a PAC (porting authorisation code) code from your existing provider and give it to your new provider and your number will be swapped over.