Well bob, the drain capacity of the tank will be determined by the number and size of bulkheads the tank has. I ALWAYS reccomend two drains. One physically higher than the other with either drain being able to cary the flowrate of the chosen return pump. That way if one drain fails, a snail crawls in it, algae overgrows it, or who knows what gets stuck in there, you'll have another backup drain. After having a snail clog my primary drain, DAMN am I glad I had a backup.
That being said, if the bulkhead is 1.5" you'll have a smaller pump than if it's 2". Wider pipe, more flow, bigger return pump. Also, the location of the sump will play in here. If it is to be below the display tank in the stand, things will be different than if it's in the basement or the room nextdoor... And lastly, if this is goin to be a 125g tank, a 29g is pushing it as far as a large enough sump (a little undersized). Furthermore, the footprint of a 29 is a pain to work with and may be difficult to fit the tall height of one inside your stand. If the stand allows it, consider something like a 30 long (48x12x12) as these make really nice sump tanks.
