I was wondering if someone with a bit more knowledge than me would be able to help.

I am trying to find the height of the numbers on the speed signs from the 60s to 80s?  I have attached an image of one to help with the type I need help with.

Cheers

I can't answer your question but can only refer to my set from West Hill Wagon Works. They measure 15 mm from the bottom of the 'stick' to the top of the figures. Obviously you can 'plant' them at any height on your layout provided they are firmly mounted. I suggest that this was much the same practice on the real railway.
I seem to recall they were once painted white, before yellow was recognised as a 'hi-viz' colour. Can anyone confirm when the change took place?

Possible circa 1963 repainted from white to yellow if anybody remembers that cold winter so the railways thought safety and visibility if that jogs a memory

Should be able to work it out from the dimensions for this complete sign?

https://www.vinterior.co/home-decor/accessories/vintage-signs/british-rail-railway-speed-restriction-sign-45-mph-sku21712555

Take a look at the photos on this ebay listing - tape measure included

https://www.ebay.co.uk/itm/284419853404

Thank you to everyone for your help, it has greatly helped.

I was in the middle of designing some and had that horrible thought I had missed something as it didn't look right, that something turned out to be the font I was using. I had the right height for the numbers however it was too wide. Have found a narrow font while retaining the simple shape of the number 1. There is a lot of narrow fonts however the number 1 is not the same style as used on the cast speed signs.

Re the change from white to yellow letters, like crewearpley40 mention, this is what the railsigns website has quoted "In 1963, following what had been an exceptionally cold winter, BR took the decision to re-paint all cut-out speed restriction signs yellow to improve their visibility in snow"