I finally figured out what's going on with the wheels. It has a 27" wheel in the back and 26" wheel in the front.
I'll post the full specs of the bike as it is now in a new thread.
Also, not sure how I didn't notice this before: the Suntour NSL High Normal front derailleur is broken and held together by a zip tie. Also, the bottom bracket shaft seems to be bent as both chainrings wobble together.
I measured the rear dropout spacing, and it's 126mm. Looks like I can use a regular 6 speed freewheel.