HTC announced the One (M8) yesterday and while there will be plenty of carrier branded options available, there is also the opportunity to buy an unlocked model. Those looking to go the unlocked HTC One (M8) route can look towards the Developer Edition or the Google Play edition. That said, those going either of those routes will need to pick up a SIM card.
More specifically, you'll need to pick up a nano-sized SIM card. Those who happen to be moving from a Moto X will be all set and can simply swap the SIM card. You could also cut a micro SIM down to size, but we would suggest taking the safer route and simply grab the appropriate sized SIM card to start.
Just like there is a variety of options for purchasing an unlocked HTC One (M8) -- there is variety of options in terms of buying a compatible SIM card. You could go straight to the carrier, or you could go with an MVNO style carrier. To that last point, Straight Talk recently began selling nano SIM cards for both AT&T and T-Mobile.
The Straight Talk service is simple, buy a SIM for $6.99 and then activate it. The service is priced at $45 per month and includes unlimited talk and text as well as 2.5GB of high-speed data. We had been using a Straight Talk AT&T SIM for a while with positive results, but have recently switch to a carrier provided T-Mobile SIM. The T-Mobile SIM, when activated online, allowed for 100 minutes, unlimited messages and up to 5GB of high-speed data for $30 per month.
Bottom line though, if you're getting an unlocked HTC One (M8) you will need a nano-sized SIM card.