A friend of mine did carb work on a BMW R75, and I spent a bit of time riding behind him on the way home from eating dinner a few weeks ago.  The rear tire was incredibly tall and skinny by current  standards - it has a 4.00-18 rear tire.  It is also common that the early rims were very narrow, 1.85 and 2.15 were common front/rear rim widths, and a lot of the early tires sizes are made to fit those narrow rims.

This summer I have been running a 110/80-18 rear tire and a 100/80, and the front looks fine - but the rear tire does appear skinny since we are used to the current fat tire look.  There is absolutely no problem with traction on those narrow tires, and several rides on the Tail of the Dragon confirmed that my 110/80  & 100/80 setup is plenty of tire for the HP and weight of the bike.  Last summer I was running a 130/70-18 tire on a 3.50-18 rim....and it looked a bit too much like a Cruiser rim/tire after seeing the narrower 2.50-18 rim and 110/80-18 tire on the bike.  It rode fine - but it is considerably heavier than the smaller/narrower tire.

My next rear tire will be a 120/80-18, and I believe that will be a good size, and it is the maximum width tire that should go on a 2.50-18 rim.  I like the 100/80 tire on the front - but that size is only available in the Bridgestone Battlax BT45, the 90/90-18 size is only slightly narrower and comes in a lot more brands.  The original 100/90 front tire looked fine with the big rear 140/80-15 donut that comes on the bike stock - but it is just a bit too tall looking (in my opinion) for the lower 70/80 profile rear tires.      

Not with anything normal sized, a 130/70-18 fits fine (25.6" OD), and a 110/80 fits fine (25.2" OD), and a 120/80-18 would be OK as well (25.7" OD)....not sure how much taller you can go.  I believe a 4.00-18 would be too tall (26.4" OD)....and a 120.80-18 is probably too tall as well (26.6").  The Vintage tread Firestones have very square shoulders and do rub on the stock swing arm. (I believe RYCA has a maximum OD size listed somewhere in their guides).

You do need to make sure the tire fits properly on the rim width you are using.

Dave: to match the front, a 120/90 is available in both 17" or 18". 17 may be a better fit.
Avon or Bridgestone.