I spent about 20 minutes tinkering around and found out some optimal sizes for your player
if you want to maximize the slap shot bonus you receive from shaft flex. It's probably only useful for D-men as it would make a forward pretty slow and does not make the wrist shot much better...
great for devastating point shots though.
In order to get a 5 star bonus to your Slap Shot Power your player should be 6'3 241 lbs. (my favourite) or 6'2 270 lbs. That is the lightest a player can be at that specific height in order to get the max bonus. As far as I know it's impossible to get the 5 star bonus if you're any shorter than 6'2.