First of all, you can just do a reverse up tilt anytime that they would be in the air in front of you, so it's not nearly as hard to hit with as you make it seem. Second, what does the character's size have to do with anything. I pull reverse uptilts all the time in melee (on Marth and Shiek no less), as a nice little way to pop the opponent up and because it's harder to DI than a normal up tilt. I'm not saying it's the end all newly discovered strategy, I'm saying that although it's a little situational, it's still not useless (it's like Marth's dash attack in melee in terms of usefulness)
How about, instead of uptilting the opponent that's in front of you, you turn around and uptilt instead? How about treating this like a situational move instead of trying to spam it?
Please do, because I want to know if they'd just be able to use their second jump right out of it, or if it would actually force them onto the ground like I want it to.
Sorry, it took me way too long to get back to you on this.
I tested it on some different characters, but not too extensively.
Basically, for it to be a spike, they have to be right at the very bottom of the last part of the attack. Most of the time the higher part of the sword hits the person's face before the lower part of it does (the part that spikes)
Five situations would allow for the spike to occur.
1 - They're short enough to get hit by the underside of his utilt
2 - They're recovering, and coming from below the ledge, but miss their sweet spot (tested a few times on Lucas, could never get it)
3 - They're tumbling and you catch them right before they hit the ground with it. (never tested, but definitely possible.)
4 - They're ducking/crawling
5 - One of their moves comes out and it misses you, but lets say it was their leg, and you could hit the leg with the underside of the utilt, and it'd spike.
This would be good on olimar, lucas, ness, and that's probably about it.
Up Smash works as a spike, which is in the hitbox that shoots out of Marth's feet, so if someone is recovering from below on a slanted stage (so the hitbox extends past the ledge) and they miss their sweet spot, it will spike. Basically it's just like a reverse up smash, but they go down, instead. (I'm pretty sure it has the same force of a tippered up smash)
I'd test to see if it could be jumped out of, but usually people don't want to do that sort of stuff, they just want to play Brawl. And it's hard to do in training mode.
One last edit: Probably not very useful on Snake, but maybe Sonic. It'd be easier to just grab Snake's cypher and send him falling in stead. I could see it working on Sonic, but it'd require perfect positioning (not on Marth's part, but Sonic's) to catch him after his Up B with it.