People say 6' min. for a tang, but I think a 150 is fine for a tang. The size of sump depends on the size of the skimmer and what you want to put in your sump like a refugium, pumps for reactors, bubble trap (baffles), room for main pump. Calculate that all in and try to get the largest sump that will fit under your stand. I have a 55 gallon under my 70 and 1/3 is for my refugium, the rest goes to the skimmer and smaller pumps then a bubble trap and out to the external pump.
“Success is walking from failure to failure with no loss of enthusiasm.”
- Winston Churchill