What kind of crankset to get for this bike?
Sounds like a poorly assembled bike to me - I reckon its a warranty job if it came with pedals installed.

If you installed the pedals yourself, then you didn't do them up tight enough, and they've stripped the threads.... though a month is quite quick for that to happen.

Are you sure the pedals and the cranks had compatible threads?  Not a 9/16" and 1/2" mashed together with force?

To answer your question - you need a ruler.  It looks like a 175mm long alloy crank with a 5 leg spider.  If the warranty doesn't cover it, you'll need to remove the crank, and take it to a LBS.  

Tools you'll need:


Crank Puller
14mm socket to get the nut/bolt out of the middle.
Whatever fitting the five chain bolts have - possibly a hex head/allen or maybe torx.


Or get the LBS to do the job, and to give the whole bike a once-over for safeties sake too.  I see in the Amazon link comments like "returned, pedal wouldn't thread in the hole."