Bigby Presents: Glory of the Giants, Path of the Giant Barbarian
3rd-Level Path of the Giant Feature
Your rages pull strength from the primal might of giants, transforming you into a hulking force of destruction. While raging, you gain the following benefits:
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Giant Stature. Your reach increases by 5 feet, and if you are smaller than Large, you become Large, along with anything you are wearing. If there isn’t enough room for you to increase your size, your size doesn’t change.
Additionally, at 20th level, you have unlimited rages.
This isn't exactly what you were looking for, but I would suggest that you change your vision to a controllable, temporary increase in size as often as you want it. It doesn't have to be from the Barbarian, but that is my suggestion. This is because a large size can be a sizable detriment to you due to the squeeze mechanic. With examples like the Path of the Giant Barbarian, you can have most of the benefits of being large, or even huge at 14th level, with little of the drawbacks.
Lastly, since you seem to be going for a more brutish focus, using a martial class instead of a magical one should fit your aim more, instead of going for enlarge/reduce or potion-making.