I have seen trophies stacked on or around cars at almost every show I've ever attended (midwest and now Texas). I don't like it and I don't understand the point of it. If the car is that impressive, it should speak for itself.
For info signs, the Texas All British Car Days show posted a template to the TXABCD website for an 11"x17" sign, complete with a British flag in the corner. Big enough to read without squinting or crowding the car but not crazy big. AACA sells an "approved" sign (with AACA logo) which is 16.5" high x 12" wide.