The seat post (and the saddle) got stolen on my Schwinn Loop. I've searched everywhere but I haven't been able to find the diameter/size of the seat post so I can look for a replacement online. Any piece of info would be appreciated.
Sorry to hear about your loss. Is this your bike?
The manual is at http://media.schwinnbikes.com/media/uploads/blog/files/S14-Folding-Owners-Manual.pdf and is utterly useless.
From a review I found "[the loop] has perhaps the longest seat-post I’ve ever seen on a bike so no doubt it will be able to accommodate very tall people with ease. " So that means a 400 or 450mm long seatpost.
As for the diameter of the seatpost - you can use calipers to measure, or take it into your local bike shop. It could be anything from 25.4mm to 34mm with 27.2 and 30.something being more common. 0.2mm difference will make the seatpost wobble and wear faster, so got to get the right measurement.
Regarding the seat (saddle) the review mentioned its a particularly horrible experience, and that the saddle should be your first upgrade. So that's positive.
First consideration is that it may happen again. So you need to plan some precautions before parking up your bike.
Pitlocks - replace the seatpost quick release with a bolt that needs a special tool to undo. Buy this at the same time you buy the seat and post.
Plain old bolts rather than Quick Releases
Downside of these is it makes the bike harder to fold up cos you'll need to carry tools.
Given its a folding bike, you might consider folding it and storing it inside from now on.
This site lists the seatpost length as 400mm, but it is wrong. Mine is all of 550mm.