You can get their normal SIM card and buy one of the SIM cutters on eBay. They sell both the normal and micro SIM cards directly on their website for $0.99 each. The nano seems harder to get....they want you to sign up for a plan to get one.
If you have iPhone 4/4S you can use micro SIM sized card. If you have a 5 you need nano SIM sized card.
I have an iPhone 4. I got the full sized SIM from T-mobile and bought a nano SIM cutter on eBay. Cut my card to nano size. The cutter came with an adapter to convert it back to micro size for the 4 (but I'm already cut to nano should I decide to get the 5 later). The SIMs are the same internally, so you don't need a specific size to get a certain speed of service. You just need to live in an area where T-mobile has refarmed their 1900 MHz spectrum to HSPA+ service. You can get up to 3G speeds on iPhone 3GS/4/4S and even faster speed on an iPhone 5 since it supports the full HSPA+ 42.