Can a monk using Open Hand Technique knock any size category of creature prone?

Question

The D&D 5e Player's Handbook states:

Open Hand Technique

Starting when you choose this tradition at 3rd level, you can manipulate your enemy’s ki when you harness your own. 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:

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

• It must make a Strength saving throw. If it fails, you can push it up to 15 feet away from you.

• It can’t take reactions until the end of your next turn.

Let me understand this correctly: a monk need only spend 1 ki point to make two unarmed strikes as a bonus action AND impose a Dex save from his opponent (regardless of size) with each of those attacks?

Let's say a halfling monk squares off against a large creature, an ettin for example. The monk spends 1 ki point to flurry, hits the ettin with the first of these two attacks and decides to impose a Dex save. The ettin saves and then the monk attacks with the second of the two attacks, hits the ettin again, and then can impose another Dex save lest the ettin fall prone? Is this right or is there something I'm missing?

If this is indeed correct then it could potentially lead to some terribly unbalanced encounters, all beginning as early as 3rd level (when a monk gains access to this method).

Answer 1

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.

Answer 2

This question was officially answered in the Sage Advice Compendium:

Can the monk’s Open Hand Technique push a Large or larger creature or knock it prone?

The Open Hand Technique intentionally ignores creature size. A monk’s ki fuels many extraordinary effects! If a feature is limited by creature size, the feature tells you so.