Surely any rail for a given length will suffer some sag. The more massive the supports then the more the sag due to its own weight it will all be down to some form of compromise between size and allowable sag. This is the reason why 8 x 4 tables are massive with many legs all supported on a concrete floor. As in most things as you go up in size the problems all get that bit harder to do on the cheap, best thing is to build them on the floor!