I dont care much for the greedy FMICs. But some poeple do ok with them. I like to use the biggest that will fit in the car, and that doesnt include length. Any time you can increase the height and thickness of a side to side core, you reduce pressure drop and increase flow capacity. Making it longer doesnt allow any more flow, and increases pressure drop. I stick to 24" length, and 3.5" thickness. Height is typically 10", or you can fit a 12" (in a 2g) if you make sure it wont over heat.
The typical sizes that DSM specific vendors offer FMICs in is the same. 24x3.5, and 10 or 12 inches tall. I like the beefier bar and plate types over the tube and fin. Tube and fin is lighter, but bar and plate is typically more effective.