If you know how to train who wouldn't want to go for SF, most valuable player. Obviously it is so much easier to train a Big or a guard, but for long-term I would def go with an SF.
SF's don't have a limit in BB nor do they have one in the NBA(ever heard of Chuck Hayes? 6'6 Center?)
But the height is important in BB for training. I would recommend anywhere between 6'2-6'10, sure they can be taller or shorter but you would want one somewhere in between there to get the most out of them. If you ask me 6'6 or 6'7 would be perfect.
Something I went with recently is trying to find guys who are like 6'9+ but listed as a PG or SG. You figure that way they should start with good secondaries and get those polished off relatively soon, and then hammer the inside skills to make a much better all-around post player than the typical tri-skilled junk Cs that fester throughout the game.
If you're not really scouting and just hoping for a random player to be a good sale value, especially if you're picking late, doing the combine and prioritizing the guys who are SF height can be a good tactic. You figure a lot of teams are going to either be looking for big men or guards so there's a decent chance that your best bet to find a guy that everyone else passed is on the 6'7 type players.