It depends a lot on the size and rigidity of the machine you are using. A good machine will cut through 18mm MDF in one pass. I think the main problem is tool deflection - a small cutter (say 3mm) cutting all the way through 18mm MDF with a decent feedrate is going to bend and probably break. Some examples...

6mm cutter, 18mm DOC, 0.02mm deflection at 740mm/min, power 290W
8mm cutter, 18mm DOC, 0.02mm deflection at 3550mm/min, power 1.86kW
10mm cutter, 18mm DOC, 0.0122mm deflection at 4650mm/min, power 3.04kW
12mm cutter, 18mm DOC, 0.007mm deflection at 4950mm/min, power 3.88kW (that's max feedrate)

As you can see using a bigger cutter will reduce the deflection enabling you to use a higher feedrate, however since with a bigger cutter the rate at which you are removing material is greater you need a much more powerful spindle. You need to find a balance between all these variables. You could run a 7mm cutter at 9mm DOC and 8300mm/min using 1.64kW. However clearly a 10mm cutter will cut the parts out a bit faster than that in one pass, so it's only worth considering if the machine isn't that rigid or does not have a sufficiently powerful spindle.

Hope that helps...there are a lot of factors to consider.