It does depend on what type (1 to 4 plus war damaged replacements), size and, thus, style of box, you are intending to model.
Firstly, two books on the subject, I can thoroughly recommend. Both by the same author, the late George A. Pryer.
https://www.amazon.co.uk/Signal-Boxes-London-Western-Railway/dp/0853615659/ref=sr_1_1?s=books&ie=UTF8&qid=1512400969&sr=1-1&keywords=by+G.+A.+Pryer
&
https://www.amazon.co.uk/Pictorial-Record-Southern-Signals-1977-03-25/dp/B01F9RC06U/ref=sr_1_10?s=books&ie=UTF8&qid=1512400969&sr=1-10&keywords=by+G.+A.+Pryer
Secondly, we, the BWMRG were able to obtain just one (To start with) decent (I.e :- we were able to count the bricks) photograph of the S/Box in question. From this, we were able to estimate the size of the main glazed panels to be approx.4' - 9" high X 2' - 9" wide* (including the framing) with the individual glass panes being 12" high X 9 1/2" wide. (In later years, some of the sliding panels [and other] had their glazing replaced by three larger, horizontal panes.
*Aaand anotherrr thing (Imitating Private Frazer) The main glazed panels did vary in height. Best refer, again, to photograph.
This is the Group's SketchUp drawing based on the photograph. As you can see, it is a large-ish seventy-one lever S/box of L&SWR Type 3 design.
All the best,
Frank.
P.S.
We are still looking out for views from the rear.