[RPG] Can you push someone upwards into the air

Yes, the monk can knock creatures of any size prone.

Other methods of proning specify their size limitations, for example, under Shoving a Creature:

The target of your shove must be no more than one size larger than you, and it must be within your reach.

And similarly, under the Battle Master's Trip Attack:

You add the superiority die to the attack's damage roll, and if the target is Large or smaller, it must make a Strength saving throw. On a failed save, you knock the target prone.

The Monk's Open Hand technique just says:

Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on the target:

• It must succeed on a Dexterity saving throw or be knocked prone.

Since it doesn't include any limitations or restrictions, there are none.

The next section is a textwall: The tl;dr is that prone just isn't that powerful.

As far as this being unbalanced is concerned, I think you're overestimating the impact of the prone condition. The effects of the prone condition are to make attacks from adjacent squares more accurate, and attacks from a distance less accurate, as well as making the prone creature's attacks less accurate.

None of this is a particularly big deal, and since any creature can end the prone condition just by spending half their movement, it's unlikely they'll ever feel the penalty imposed on their attacks. In effect, the prone condition merely increases the accuracy of attacks from adjacent squares until the creature's turn, then costs them half their movement.

Furthermore, for this feature to work, it requires a successful attack followed by a Dexterity saving throw. The target creature effectively gets 2 chances to defend against this effect. It's also worth considering that some monsters (for example, elementals) are immune to the prone condition, so this simply won't work on them at all.

Finally, remember that ki points are the Monk's main resource, and they are quite scarce, especially at low levels. Spending ki points is meant to have a significant impact, otherwise they wouldn't be quite so rare. If the effects you got from them weren't useful, the Monk class would just be a weaker version of the Fighter.

The Ki power Flurry of Blows, is very simple:

Immediately after you take the Attack action on your turn, you can spend 1 ki point to make two unarmed strikes as a bonus action.

Per Melee Attacks, an "unarmed strike" is a "melee weapon attack". Making an Attack includes three discrete steps, the attack roll is one of them.

Unless it says otherwise, multiple attacks are just that - multiple attacks, not a single attack that deals damage more than once. If there aren't two attack rolls, it isn't two attacks - it's one. Each attack must be rolled separately and hits or misses on it's own - anything that triggers off the hit (such as damage) is likewise resolved on its own. Extra Attack works the same way.

Way of the Open Hand's Open Hand Technique says:

Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on that target.

"Whenever" and "one of the" are the key bits of phrasing here. The two attacks don't have any effect on each other. If both attacks hit, you can inflict a different effect for each hit. In fact, you should resolve them individually and sequentially. If you knock the target prone, you have advantage on the next attack. If you knock the target away with the first attack, you need to use movement to make the second.