I'm re-posting this in a new thread to get it pinned. This question pops up every so often and at least one person thinks this answer has some merit (thanks cupieqt), so here goes...
I tend to hold the (less than popular) opinion that size does matter... when all else is equal. That means that given two athletes, both have the same level of experience, flexibility, strength, body awareness, explosive power and nerve; the smaller, lighter one will have more overall success with stunting, especially co-ed style.
However, life isn't like that and you can rarely, if ever, make such an apples-to-apples comparison between two athletes. There is nothing you can do about your height and aiming for a body that is athletically fit is far preferable to try to hit a specific number on the bathroom scale.
In my ideal world I would be significantly taller and bigger. That would be of significant advantage when partner stunting. Again, I can't do anything about my height but I can hit the gym and get as strong as possible for my size and frame, so that's what I do.
I've included a video clip of me doing a rewind with Victoria. She's actually taller than I am but after practicing with her 1-2 times a week since September (roughly 4 months total) we can do a fair number of decent skills; rewinds, full-ups, switches, double-downs, hand-in-hand, etc. That's the dedication and focus. She really works hard at being centered over my hands and lets me do my part in keeping her up there. I swear that any time we do tick-tock skills I could close my eyes because she always switches feet back into the same point in space -- directly into my waiting hands.
So, if you look at our physical stats they are something like this:
Me 5' 7" (67") and 180lbs
Victoria 5' 8" (68") and 120lbs
relative height: 100%
relative mass: 66%
That's fairly unusual for co-ed stunt partners. A more typical example (at the elite levels) is shown by @kingston in his video with Katie.
Not trying to call you out or anything there big guy, just trying to show an example. So, from looking at his video I would guess their stats to be:
Kingston 5' 11" (71") and 215lbs
Katie 4' 10.5" (58.5") and 95 lbs
(Turns out I was close, just a little off on when I guessed Katie's height to be 5' 1" -- that would have made her a giant )
relative height: 82%
relative mass: 44%
For me to match their elite awesomeness I would have to try to find a partner who is 4' 7" (55") tall and 80lbs to match the relative size/mass. I actually know someone close to that size and she's a lot of fun to stunt with but she prefers to cheer all-girl (sigh). I can do rewinds to cupie, one arm rewinds, double ups and all kinds of crazy stuff with her -- and that's just stunting with her a couple of times a year. I can only imagine what we could hit if we actually worked at it.
Alternately, we could see it wouldn't be as easy for him to stunt with a partner who is 5' 11" and 140lbs. I'm not saying they couldn't stunt together, just that they would be more limited in what they could achieve. kingston has pointed out that his cheer partner in college was 135lbs and he did full ups and rewinds during games with her (remember boys and girls, don't put up anything during a game that is not 100% guaranteed to hit -- I'm pretty positive that he lived by that same motto).
For the record, I never doubted that kingston can stunt with just about anyone at a high level and can probably toss anyone (many guys too) to hands and put them in a liberty, as long as they stay tight and don't freak out. I'm just pointing out that the bigger the size gap, Big Guy -> Tiny Girl, the easier it is to stunt at the elite level, e.g. multiple twists up, one arm stunts, combinations and transitions and many stunts done in a row. Let's face it, if your partner weighs 20% less then it stands to reason that you can do 20% more stunting in the same time period between rests. Heck, if she is small enough you don't need much of a rest at all. And of course the more (good) reps you get in, the faster you both improve.
It's not popular to say, but we can't be everything we want to be; no matter what the motivational posters and our mom's tell us. We CAN be that which we work at diligently and for which we have some natural aptitude/talent. In some respects we can make up for the lack of natural skills by working extremely hard but there will be limits. At my size I would never have been a pro basketball player no matter how long I trained or how dedicated I was to the cause.
let's go back to the original poster's question that started my reply... is 5'4" and 120lbs too big to fly? No, but you may be more limited in what you can achieve than someone who is 4'10" and 85lbs.
Whatever your size, take a realistic assessment of your build; are you athletically fit and strong or soft and flabby. Change what you can, improve flexibility and body awareness and decide to what extent you can dedicate yourself to becoming the best you can be in this activity. And it never hurts to look for big, strong bases to put you up in the air. And for every girl in the air that thinks she should be smaller, there are bases on the ground that think they could be bigger too.