Hi Eddie, the simple answer to you question is no there is no mandatory size for health & safety signs but there are sort of standard sizes.
If you contact Signs & Labels on: 0161 494 6125 they will send you a catalouge and price list, allsigns do the same but I cant find their number.
If I need a lot of signs I usually order them from someone like this, if its only a few then I make my own. As for substrate if its outside I use either 3 or 5mm foam depending on size if its internal and they want ridged I use HIPS. Having the catalouges from the suppliers makes it easy to price the ones you have to make yourself.