For a machine this size I,d have bearing plate 225-250mm. This gives a good balance between ridgidity and maximising the travels.

Regards your first post and you mentioning 3200 resolution then let me just clairy something that you may not be aware of.? (Think I've said this before, if not I started writing then got disturbed.!!...Lol)

Don't class micro steps as resolution because it's false, MS are mostly to help with smoothing the motor rotation and they do come at a cost in that you lose torque and require a much higher number of pulses from the controller for the same RPM causeing it to work much harder and with more chance for missed steps. Also much above 2000Ms is wasted on most steppers because they cannot resolve to those fine amounts. 1600 MS is more than good enough for most stepper systems.

If you want higher resolution then either buy smaller pitch screws or use a gear ratio then this will be your true resolution that can be trusted 100%.

Regards step errors and closed-loop then again don't rely on the closed-loop.? Step errors are often caused by over tuning the motors or binding axis etc and this is no different for a closed loop system.! The errors will still occur but the encoders will ensure the motors catch backup at some point. However the error will still have occured and will possibly show on your part, depending on what it is your cutting. The only way to ensure that you don't get step errors is to size and tune the motors correctly.
When tuned correctly and provided everything on the machine is aligned and sticktion free then you will never get step errors on a correctly tuned machine and it will perform just as good any Closed loop system that use the same type of motors.
Where Closed loop systems differ and are better is in the fact you can get 3 phase motors with a finer 0.9 or 1.2deg step angle, these run much smoother and provide more power at slightly higher Rpm's because of being 3 phase.

But if you have Steppers already that are the correct Size then with good Digital drives and correct PSU they will match any 2 phase closed loop system on performance and accuracy provided they are tuned correctly.