To make the drum rotate in only one direction you would need to disable the circuitry that drives forward and reverse motion.
The size of the capacitor in microfarads should be the same as the original or at least a very close match - motor start/run capacitors often have several capacitors in the same can and sometimes have a picture of any interconnections printed on the outside.
The physical dimensions of the can are of no consequence other than the ability to fit it safely in the enclosure.
The Voltage rating should be at least the same as the original and there's no problem if the voltage is greater - but it should never be less.
Peccavi, October 2009
