Is this a Janet Airlines 737 at Area 51?
Probably yes.

Janet Airlines operates the Boeing 737-600 (and some Beechcraft turboprops), which has a wingspan of $ 34.32 \, \mathrm{m} $ and a total length of $ 31.24 \, \mathrm{m} $. I measured the size of the aircraft using the Google Maps distance tool:



The dimensions of $ \sim 34 \, \mathrm{m} $ span and $ \sim 30 \, \mathrm{m} $ length match (within the accuracy of the Google Maps image) the dimensions of the 737-600. The white fuselage paint also matches the Janet airlines aircraft:

(image source: Wikimedia)

Most civilian aircraft are painted white of course, but military aircraft are typically not. The red stripe is not visible from above, so it is impossible to say for sure, but since I don't expect many other white 737-600s to fly to Area 51, I think it is a fair bet to say this is indeed a Janet airlines aircraft.