There are two types and three sizes of SIMs to choose from.
Prepaid activation kits, which include a SIM and an activation code, are available in mini/micro combo and nano sizes. The activation code is required when activating a new line of service on any T-Mobile prepaid plan including the $30/month 5 GB of data, unlimited messaging and 100 minutes plan, which is only available with new online activations.
Mobile broadband SIMs in mini, micro and nano sizes are also on sale for a 99¢ with the SIM99 code. These SIMs can be used to activate a tablet or hotspot on any T-Mobile Prepaid Mobile Internet plan. They also let you activate a supported tablet on any T-Mobile Mobile Internet plan including the 200 MB/month free plan.
Either type of SIM can be used as a replacement SIM for any T-Mobile account postpaid or prepaid.
There's a limit 3 SIMs per per order and 5 SIMS over a period of 30 days.
Related post: Prepaid Operator Profile: T-Mobile