Here is a pic from over at Loose Rounds article entitled The Disappointment that compares Glock 42 and Glock 26 side-by-side. As you can see – and as I said earlier – there is not much of a size advantage:
Yes, its a single stack so it is indeed slimmer (more at the link), but its still too long (0.47″ shorter than Glock 26) and still to tall (0.04″ shorter than the Glock 26) to be considered for true pocket pistol use. Its gonna still have to be carried in a holster, so unless you are severely recoil adverse just go all the way up to a big boy caliber and carry the Glock 26.
For the record, I too am very disappointed that The Glock 42 was not the 9mm Luger it should have been.