I have been scanning and measuring all my Bath handstamps as the first step in working out which ones match the entries in the British County Catalogue and which ones don't. I have been using an old version of Photoshop but one can do this with free software (eg. GIMP - a different interface from Photoshop which I am used to, so I haven't switched).
Using these software tools one can zoom into an image to measure it to the nearest 0.1 mm - in practice an accuracy of 0.3 mm is about as good as one can get given the normal clarity of the handstamp strikes.
Here is an entire from August 1754 with a nice clear "BATH" straight-line handstamp - when I measure it using the software tools I get a size of nearer 28 x 7 mm than the 27 x 8 mm that is in the BCC (the measurements taken without the serifs as I understand was the standard for the BCC).
The entire is franked "R. Allen" and is probably from Ralph Allen's nephew-in-law.
Ralph Allen died on 29th June 1764. Here is the inner sheet of another letter from him, headed Prior Park 11th December 1763 (there are no postal markings).