Honestly, the abilities partly depend on the situation. Not every environment allows for a Colossal sized creature, for instance. The phasing abilities would be nice for defense, mostly, though I'm not sure how often that's a plausible option. I couldn't pass through a whole planet since you need to be within 5 feet of the surface, but it could allow walking through walls. In fact, everything it allows me to do is replicable by Fighting styles, though frankly at later levels.
Sizing isn't something I can replicate with fighting styles, but eventually I'd just be able to take Huge Size, and Enlarge Person would get me up to Gargantuan. Sizing essentially just saves me a feat, unless of course it's ruled that changing sizes with Sizing doesn't work the same as changing a creature's size. If that's the case, I'd probably want to take the feat anyway. However, I have been hoping to find a way to both use Harmless Form faster and add to its versatility. Sizing also lets me shrink to smaller sizes for utility purposes. But allowing these abilities do break or bend the rules of the traditional game. Perhaps not more than a wizard can, but still.
I'm not trying to open a can of worms here. Just want to know the boundaries. You've got final say on everything, and can always call shenanigans on something if I or anyone crosses the line. You could also set up ground rules, set up an official ruling on what benefits one would and would not get, etc.
As for the Magic Clothes, that's what I figured. I can add the abilities I otherwise would have to mundane/less expensive clothes and forget about the +2 deflection bonus to AC and +1 to cha skills (which honestly isn't as nice as the AC boost, which again isn't imperative), or live with lesser enhancements. Not a huge deal either way.