See ad:
http://portland.craigslist.org/yam/bik/755292713.html
Anybody know anything about it? Just curious.
it may be a 42".
if not then it’s just a coker.
lolz… trade for a road bike!
my guess would be a 42’’, but considering its older, it might be a weird old 36’’.
Nice picture. 
I have never seen anything like it
60 spokes ! Almost certainly a hard wheelchair type tire.
It’s an old-school big wheel, most likely from the Unicycle Factory, Tom Miller. Those were (almost all) 40" wheels. It will have a serial number and a date stamped into the lower part of the frame, saying when it was made (or finished; sometimes there was a long gap), and indicating how many big wheels he’d made before that one. I have #30.
The tire is wheelchair rubber. This type of big wheel is great for shows and parades, but not as great for long rides as the tire is very hard. They love to swoop and spin, they don’t love bumps! Yes, the seat looks like a replacement. The original was most likely a Schwinn.