Hippynerd:
Lets use 20 mA x 64 LED as an example. Assuming its running on 5v, using constant current driver to sink, (that sounds like the direction he is heading), it should use up to 1.28A. With this design, he is only doing 12.5% duty cycle. How do we apply the math to figure out what specific specifications to look for, and ranges maybe? It seems like a simple table would be easy enough to make.
Let's look at power dissipation first. If we assume that the software works so that the 1/8 duty cycle won't be exceeded, and we go for 0.5W maximum power dissipation in the mosfet, then the dissipation while the mosfet is conducting can be up to 8 * 0.5W = 4W. So we need Rds(on) <= 4/(1.28 * 1.28) = 2.4 ohms.
However, the voltage drop would then be 2.4 * 1.28 = 3.1V, and we certainly can't tolerate that in a LED driver running from a 5V supply as there wouldn't be enough voltage left to drive the LEDs and series resistors. So in this case, maximum Rds(on) is determined by maximum allowable voltage drop. If we were not concerned about the LEDs getting dimmer as more are turned on, we might allow 0.5V, allowing Rds(on) of up to about 0.4 ohms.