If you aren't familiar with just how cute the M.2 form factor is, the picture below shows it attached just below a SATA power and data connector which, when you include the plugs, is significantly larger than the 128GB M.2 SSD.  This allows you to slip this style of SSD into just about any sized device you could want as long as you have an M.2 connector.  The KingSpec models that The SSD Review just benchmarked use a JMicron so aren't the fastest on the block but that has nothing to do with the size as these top out at SATA 6Gbps just like any other drive.  Check their full reivew here.

"The KingSpec CHA-M2B7 M.2 NGFF ultrabook SSD is available in capacities of 32, 64 an 128GB and is a 42mm(L) X 22mm(W)X3.2mm(H) SATA M.2 SSD. To say it is small is an understatement as we can fit four of these on a typical business card with plenty of room to spare. "

Here are some more Storage reviews from around the web:

Storage