If you want to be sure of the diameter , take a piece of tie wire (or whatever), curve and lay it in the channel where the glass fits so its touching the outer ring of the channel- mark it and then cut it exactly where the overlap occurs. Take it out, straighten it and measure the length. Then divide by 3.14 . The number will be the widest diameter possible. It'll probably be something like 26.70 or so. Your 26 1/2" cylinder would be 26.50 ,The .20 (Or whatever) is the slop for the sealer (litharge)they would put in at the factory.
Measuring for the height should be self explanatory