prestonj:
I need it to be as small as possible
The easiest way to obtain SMS capability would be to use a cellphone shield, but that means your device has the footprint of a UNO. It's possible to get much smaller Arduinos (down to a single 8-pin DIP chip if you like) but you would be ruling out these smaller packages if you are using a standard shield. So, I suggest you need to decide how important 'as small as possible' is to you and what size is actually acceptable. If you're aiming to make it smaller than a UNO then you have a compromise to make between size and difficulty.