as far as demo: dunno about TSR, but most places that demo will do so for a small fee (20-30 bucks) per demo and will credit the fee back when you purchase. just tell them you wanna demo a board and ask how much and whether they'll apply the fee to the purchase. just be nice and hopefully they'll hook you up!
as far as size: the reason there's so much variance in the "recommended" size is that it depends on preference. some riders like larger boards cause they'll generally provide more pop and ride highger in the water and be faster than the same board in a smaller size, but the smaller size will generally be lighter and also track better (cause it sits deeper in the water). but the best thing is to demo if, possible. also, when talking about the size of the board the important factor is not just length, but total surface area, which is roughly length times width.