Toby.... a simple Google search on RN Castle Class .....then open the Wiki link
From this you will find a table on vessels........then a sub table on RN vessels of this Castle Class complete with the Pennant Number
Just where are you looking ???
Knowing what the numbers should be is useful. However, the question was "What Size should they be?"
I did some numbers for a USN ship by using a colour laser to print on white sticky back vinyl, then cutting round after placing them. It was a PITA to do on a 1:125 model, might be easier on a larger one. I could have just stuck the plastic on and removed the numbers, then used the plastic as a stencil mask and painted the numbers.
There is plenty of documentation for USN giving painting specifications, but I haven't found anything corresponding for RN. The best bet would be to have a side-on photo and do some careful measuring of something of known size on the ship and the numbers, then working out the right size and aspect ratio.