I don't want to get hung up on this transformer voltage business (except that I was surprised by the toroidal transformer I bought recently that measured about 5% above nominal voltage when off-load - the "regulation" factor that's mentioned above which I hadn't expected) but I am interested in the practical choice of smoothing capacitors. I am assuming here that the linear PSU is feeding only the stepper drivers and nothing else, which I think is generally the case.

Stepper drivers can cope with fairly wide voltage ranges. From memory, the EM806 which I am using is rated at 24-80V absolute min/max, for example, although the Leadshine recommendation is to stay at least 10% away from either extreme. We are not building audio amplifiers here, so we're not interested in hum levels on the output. The capacitors are being charged at 100Hz (assuming the usual bridge rectifier) but the steppers are pulsed at a few KHz, so any given pulse will see only the variation in voltage during that pulse - small compared with the variation across a half-mains cycle? On top of all that, one of the stepper driver's main jobs is to control and limit current through the stepper coils, so one thing it can do very well is cope with a range of input voltages while trying to maintain the target current through the stepper coils. So do I really care if the DC output voltage sags by a few volts, across a half mains cycle? In practice, then, just what size capacitor will do an adequate job without going OTT? What is the effect of a too-small capacitor? Don't know the answer myself, but once my machine is back running I'll stick an oscilloscope on the PSU output and see what's happening, just in the interests of science.