Narrow and long is the stronger option so if you work on 1/4 -8x4 sheet size then 700 x 1300 is good cutting area and you'll just about get away with using 16mm screws.
The speed you get from 1605 will depend on a few factors, mainly the size of steppers your using and the voltage your running them at. Voltage is key to performance from steppers but everything else like drives etc has to follow suit and match to get best performance.
Now it basicly boils down 2 options you'll have here.? To use drives that take upto 50V or drives that take upto 75-80Vdc. If you used 50Vdc drives then you'll run approx 44Vdc PSU and with 80Vdc drives 70Vdc PSU. These will yield approx 3500mm/min and 5000mm/min of safe usable feeds with 1605. This does depend on other factors so only a very rough guide, in practice you could tweak out more rapid speeds but won't be reliable for cutting.
Now problem with 5mm pitch and soft material like MDf wood etc is that the correct cutting feeds for finsih and tool life are always at your upper limit of speed range or above and this leaves you in funerable position for missed steps and stalls due to how torque drops away as RPM's rise.
So with 10mm pitch you still have the speed but now your motors are spinning at lower RPM for the same cutting feedrate but with much more torque available for when things get sticky etc so no steps get lost or motors stalling.

Resolution is still more than enough for cutting aluminium unless your wanting to cut very high detail, in which case your probably building the wrong design of machine anyway.!
To give an example of the kind of detail 10mm pitch will get you see my aztec calender.