Shirley's Imperial PDF gives the smaller sized one as 4" and it's not the same style as yours, so I'm not convinced they are right. I think yours might be an L.E. Smith Glass Company one, described by Shirley as I'll send you a scan by email to compare with.
I'm not sure those sizes exactly tally with mine (maybe due to a lack of understanding imperial measurements)
Mine is: Base = 84mm x 61mm (To the outside of rim) Top = 73mm x 54mm.
I will drop L.E. Smith Glass Company an email and see what they say. Thanks for you help Anne - I will keep you informed.