Size is important (?)

[snowy1051]

Hello, is it my imagination or what? Is the BRITISH RAILWAYS on the side of Bournvilles 42338 smaller than the numbers?

Then there is 42400 with BRITISH RAILWAYS super large and not on the same line as her sister. According to BR database info 42400 was fitted with 8" numbers and 6" letters in late 1948 at St. Rollox. A year later she was in Kilmarnock works and in this 1950 picture of her at Glasgow Central one can be certain then, that she was fitted again with the same livery but with larger numbers etc. My query is, was there a set rule for size of numbers letters etc. or was it random, especially on the "bread and butter" locos? my layout is set around 1953 so that livery and the "lion and wheel" are both used. 

Thanks for any info re number and letter sizes.