Here's a handy tire size calculator that you can compare tire sizes with.
http://www.1010tires.com/TireSizeCal...?action=submit
Diameter is the height of the wheel rim the tire goes on. So, no, a 16 inch tire won't fit on a 15 inch rim.
Some other issues you might want to look at are new rear shocks, maybe something like Monroe's Sensatrac Load Adjusting Twin-tube Shock Absorbers. I intend to put a set of those on my T&C.
Another option would be putting on Timbren rear suspension enhancement kits:
http://www.etrailer.com/susp-1994_Ch...nd+Country.htm