Short = about 1 inch at about a 1/2 second pacing.
Why? If you're using a DE razor, you want to stay with the flat facets of the face (like a diamond) as much as possible. If you're using an M3 that aspect is less important due to the pivoting head, but you still want a fairly short stroke for control purposes and to help avoid clogging. This video and this video may help.
The way I look at it, with a DE or a straight razor, you are trying to actually dig out the hairs while cutting them. I actually sort of curve or arc my short strokes to mke little digging motions, especially with neck hairs. The theory behind multiple blades is that one pulls the hair out while cutting, then the other cuts the remainder -- so a long stroke accomplishes the same as multiple short strokes with the DE or straight. Nice theory, but my DE shaves are closer than my M3 shaves used to be!
The DE doesn't have a pivot on the head like the Mach 3 and others that will automatically conform but in this they don't get as close, but they will save you time. You have has too many sharp angles, even if they aren't easily visable, that a fixed head can nick, but when you use very short strokes it's so much easier to stay on the contour and a very close, nick free shave.
I'll admit though as I've been using a DE for a few weeks, I often slip up and use the long strokes I used for years. Oftentimes I pay for it, at the very least my shave isn't as close.