Browsing the A4 section in RCTS Part 2A offers a wealth of information in terms of the types of lettering and numbering applied over time, especially where the name was not on a standard nameplate. Otherwise and of particular help, it states that the standard nameplate had the lettering in Gill Sans light, save for new plates fitted to 60013 in October 1952 which were in Gill Sans medium. The nameplates were 8in deep and varied in length from 3ft to 10ft, the letters themselves 5.5in high.
Another element of course may be the coats of arms on the cab sidesheets of those named after countries.
The Thompson and Peppercorn nameplates would also have conformed to that specification, although several of the latter's appeared after Nationalisation.
Edit to include height of letters.
LNER Locomotive Font
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