Size doesn't matter.
Interest does!
Seems to me that a great short par-4 could have a green of any size, from tiny to immense -- so long as the green is interesting: narrow, shallow, heavily bunkered, wildly contoured, dramatically sloped.
Mix and match.
Dan,
I agree with your assessment.
I would add that the chance of being rewarded for taking the risk must be at least commensurate with that risk. I remember seeing Jack Nicklaus sitting in the booth during one of the broadcasts of the Memorial. He said he had originally designed the 14th hole at MV to be a drivable par 4. He was genuinely surprised that no one ever tries to drive the green. I'm not. My god, the infintesmally small odds of pulling that shot off have to be astronomical.
I reviewed the new course at Cache Creek in NorCal recently. The 8th hole there was also designed as a drivable par 4. The consensus of the group playing was that no one in their right mind would attempt to drive that green more than once. Here again, the odds are so stacked against a successful drive that there is very little incentive to try it more than once.
To me, what makes the drivable par 4 work is that it must be enticing. To be enticing, the hole must present either a real or IMAGINED possibility of success, even after having failed once.
In both of the examples I have cited, there is little visually to actually enchant, bewitch or beguile the golfer to overestimate his own abilities.
At Cache Creek, once a golfer has seen the green complex, it becomes quickly apparent that trying to drive that green is sheer folly. True, the green in this case is relatively small; but that is not what causes the hole to fail as a risk/reward drivable par 4. The only "landing area," which is right of the green is miniscule, (and I mean "postage stamp" minsicule), and the carry-bunkers encroach on what little area there is between it, a back right bunker and the green. There is little in the way of contouring to nudge a ball from that small area onto the green. There is absolutely no room on the left to bring a shot in from that direction. The obvious play is to hit a mid-iron off the tee and knock a wedge onto the green. What could have been an electrifying hole is consequently quite a disappointment.
So, while I do not think that the size of the green itself matters when desigining a drivable par 4, I do think there must be some combination of design elements that convince a golfer, rightly or wrongly, that pulling the shot off is an actual possibility.