Those who follow the Way of the Diminution know that size is not always what matters in a fight.  These monks learn to channel their ki inward to condense the atoms in their body into a smaller form, while maintaining a powerful punch.

Diminutive Form

Starting at 3rd level you can channel your ki inward, to condense your form into a tiny size, while maintaining your mighty power.  As a bonus action you can gain the following benefits for one hour, or until you chose to end the effect early as a bonus action:

• Your size becomes Tiny and all objects worn or carried match your new size.
  

• Your size cannot be increased by any means until you exit this form.
  

• You have advantage on Dexterity (Stealth) checks. 
  

• You can squeeze through spaces as small as one inch.
  

• Moving beneath enemies is not considered difficult terrain for you.
  

• Enemies make attacks of opportunity against you at disadvantage.
  

• You gain no benefit from your unarmored movement. 
  

While in this form your weight does not change.  Once you have used this feature, you can not do so again until you finish a long rest, or spend 2 ki points.  

Diminutive Manipulation

Starting at 6th level you can channel your ki into objects to reduce their size and weight.  As a bonus action you may force a nonmagical object’s size to be halved in all dimensions, and its weight reduced to one-eighth of normal. This reduction decreases its size by one category - from Medium to Small, for example.  This transformation lasts until the end of your next long rest.  An object can only be affected by this feature once.

You can use this feature a number of times equal to your proficiency bonus and regain all uses at the end of a long rest.

Release Diminution

Starting at 6th level, when you are holding an object under the effect of your Diminutive Manipulation that is one size category smaller than your own, you can make a ranged weapon attack (20/60) with the object.  The object is considered a monk weapon for you.  If the attack hits, you may use your reaction to return the object to its normal form, dealing additional damage based on the original size of the object.

Enlarged Form

Starting at 11th level, you can reverse the direction of your ki to allow you to briefly enlarge yourself, instead of reducing yourself.  As a bonus action you can enter an Enlarged Form and gain the following benefits for 1 minute, or until you chose to end the effect early as a bonus action:

• Your size becomes Huge, and all objects worn or carried match your new size.
  

• Your size cannot be decreased by any means until you exit this form.
  

• You have advantage on Strength (Athletics) checks.
  

• Moving over enemies two sizes smaller than your own is not considered difficult terrain for you.
  

• Enemies have advantage on attacks of opportunity against you.

  
  

• Once per turn when you hit with an attack in this form, you may spend 1 ki point to deal an extra roll of your Martial Arts die.
  

Once you have used this feature you can do so until you finish a short rest.

Diminutive Collapse

Starting at 17th level, you can shrink your form to such an extent that it causes a small collapse in gravity.  When you are in your Diminutive Form, you can spend 5 ki points as an action to shrink until you are in the Ethereal Plane, where you remain until the end of your turn.  While in the Ethereal Plane, you may take any bonus actions available to you and move as normal.  When you return to the plane you originated from, you return in your regular Diminutive Form.While on the Ethereal Plane, you can see and hear the plane you originated from, which is cast in Shades of Gray, and you can't see anything more than 60 feet away. You can only affect and be affected by other creatures on the Ethereal Plane. Creature that aren't there can't perceive you or interact with you, unless they have the ability to do so.

When you enter the Ethereal Plane, you leave behind a tiny black hole in your space.  Creatures within 30ft of this space must make a Strength saving throw or be pulled 15ft toward it.  The black hole then explodes, unleashing a torrent of destructive energy.  Each creature within 10ft of the black hole's space must make a Constitution Saving Throw, or take 3d10 force damage on a failure, or half as much on a success.