I've got this Mountai Bike, Pretty big Considering I'm 6'4 now. The problem is, I see people popping wheelies like no big deal, yet only if i stand up lean back and pull with all my might, i only get in an accident or barely get it off the ground. I know mountain bikes aren't designed for that, but i wanna do it atleast once and i dont have a light trick bike to do it on. It's heavy, so don't assume im not trying xD.
Tips On Popping A Wheelie On A Mountain Bike?
Bassically dude im gonna tell u the same thing except once u pop the bike in the air keep the front tire high if ur start to fall backwards tap the back brake. When popping a wheelie always have one finger on the back bake or two if u like, so if u do get to the moment like ur gonna fall back tap the back brake to brign ur front tire down. The rest is just practice man mixing pedalling in with braking. Oh and practicing ur balance.
yo the fact that your tall has nothing to do with it haha 
all you hace to do. is... go down to the lowest gear possible. slow right down (nearly to a stop but just coast). then as you pull up pedal really fast lol hahah.
this is bound to work. you can lean back if you have too but im sure you wont need it.
and then just play arround with it haha.
youll learn how much pedaling you have to add and such.
and by the way i find its easyer to do standing up. try doing it sittin down later on. its really different.
haha hope this helps. im no expert hahaha!
-Eric
all you hace to do. is... go down to the lowest gear possible. slow right down (nearly to a stop but just coast). then as you pull up pedal really fast lol hahah.
this is bound to work. you can lean back if you have too but im sure you wont need it.
and then just play arround with it haha.
youll learn how much pedaling you have to add and such.
and by the way i find its easyer to do standing up. try doing it sittin down later on. its really different.
haha hope this helps. im no expert hahaha!
-Eric