OK as far as specific distances go, what I'm using to practice with is going to Dreamland in practice mode and manually pushing Fox off the left side of the left platform so he's on the base platform. A little farther is fine too. The length is about the size of a character. Definitely not room enough to follow up with a ground attack like u-smash.
I would probably instinctively go for a Fair, but that's just because I'm aggressive without thinking.
Probably a better option would be bthrow then techchase.
Or maybe just fthrow alone, and edgeguard with bair, etc.
But I can see your point...it's definitely an awkward situation
The problem with Fair is that, unless you jump at just the right time (or have ample running space, which you shouldn't), you're only going to connect the second hit. However, when you do connect the second hit, you can safely follow up with an Uair to UpB.
B-throw to try techchasing might be OK (need to try it in real matches to see how effective it is), although it would be less reliable on the edges of Hyrule. Just trying to edgeguard off the bat probably isn't a great idea, Fox isn't particularly easy to edgeguard when he's so close to the edge and has his second jump (although I admit I need more practice with CF's uTilt to really say this). Anyway, it's optimal to get at least a hit before going back to the platform to edgeguard. There's got to be a better option.
what about nair? or Dair?
edit:
I would probably try to do a fair/uair to dair/nair or something... if it does work...
U-air is too slow to follow up with, Dair's hitbox isn't high enough to get Fox before he can retaliate, Nair seems better but it's not a true combo (however, I'm not 100% sure if Fox can retaliate. He might be able to with his reflector).
dash pivot uair bair bair upB ( or pivot uair bair dair) ? Just a flashy idea, but I think it should combo.
Pivot u-air is too slow and unreliable.
If your far enough away to up smash, up smash
If you are close enough to go off the stage and rising up air (into up b) then do this.
Definitely far enough away so that you can't up-smash.
Rising Uair to UpB doesn't work, it's too slow.
If you are right to the ledge try one of those
-fair to nair
-fair to dair
-fair to uair to up+b
The last one is kinda tricky with very few space to start running.
Fair to Nair seems to be the best option. Fair to Dair is too slow (because of the locatino of the Dair hitbox). Fair to Uair to UpB doesn't work because the knockback and stun from the Uair isn't enough to be able to follow up with UpB.
first two sound impossible to land
the 3rd one is good most of the time but f-air can be DI-ed out of if they DI inside really hard
and you don't need to run.. he's at 0 percent
if you want to be fancy try doing f-throw, turn around b-air to b-air
Too far away to hit the second b-air unless opponent makes mistake.
Fthrow --> Pivot Bair--> Top hit box Uair
yum yum
Pivot is very hard with such a small space.