So obviously the character builder will have it's limitation on character sizes, as it should. Those limits have to be in place for gameplay. You don't wanna have giant group members in tight enclosed spaces making things difficult for your party. Which is why I wanna suggest Size Manipulation Pool Powers(or whatever the side powers list is called to your main powersets.)

Powers would be:

1)GIGANTISM: A toggle power with certain restrictions in place. Once toggled your characters overall mass grows to the size of say... Giganta or Antman(Giantman). Approx 50ft tall. While toggled all your normal attacks would be locked out and replaced with a handful of giant based attacks to use. These attacks would each have 2 different animations depending on the size of enemy your attacking. I'm thinking you could possibly TAG other gigantic's or large world bosses with the Gigantic Tag so the power knows which animation to use. For attacks I'm thinking things like:
Regular Enemies: Stomp(lift one foot and drive it down in front of you cracking the earth temporarily) / Gigantic Enemies: Straight Knee(drive knee into the enemies midsection)
Regular Enemies: Slam(Slam both fists down to the ground in front of you) / Gigantic Enemies: Double Axe Handle Slam(Overhead slam with both arms to enemies head/shoulder area)
Regular Enemies: Backhand Sweep(Bend down and backhand your enemies in a cone area in front of you knocking them back) / Gigantic Enemies: Haymaker(punch your enemy so hard they fall backwards unless knock resistant)
Regular Enemies: Toss(bend down and pick up your target and then throw them(enemies could take fall damage or impact damage against buildings)) / Gigantic Enemies: power grayed out when targeting Gigantic Enemy
Both: Pick up stationary cars and hold them until pressing button a second time to throw it at targeted enemy.

The restrictions on Gigantism: The power would be grayed out while in indoor maps. You also have to have an instance based restriction of up to 5 active Gigantism toggles in any given instance. If there's up to 100 players in an instance, 5 can have this power toggled. If 5 have already toggled it, it will be gray or possibly red so people know they have to find a new instance if they want to use it. I feel like 5 is an ok number per instance. If it becomes a problem number could be reduced. Seeing a gigantic being should be a special and rare sight, we don't want a whole Attack on Titan reenactment.

Gigantism could also have it's own set stats or effect certain stats of your character: +Strength, +Resistance, -Dodge, Hitbox increases

Lastly, walk and run speeds have to be set appropriately. Giants would be able to move around much faster than a normal sized being. Unless that being had superspeed obviously. lol

2)MICRO-ISM: A toggle power without restrictions. I'd like to see this power designed as a cyclable toggle(offering 3 different miniature size types depending on how many times you press the toggle button. You would basically have to cycle through all 3 before the toggle would shut back off.) The best I could describe the sizes would be... Infant, Pixie,and Bug sized. I can't see why all normal powers shouldn't be able to work with this toggle active. Hopefully props can also scale down with character. Sure power emanations might look funny, but let it be up to the player to say weather they can deal with it or not. Character hitbox shouldn't be altered by character size. Hitbox would still be the size of a small basic character.