I've determined just how big this character could become. Yellow Musk Creeper starts at Medium and goes to Huge; Titanic Creature can then grow another size category every 4 HD (for a 20-level build, that's 5 more size categories), for a total of Colossal+++. A Colossal creature's largest dimension is typically 64-128 feet, so a Colossal+++ creature can expect its longest dimension to be between 512 and 1024 feet - between 1/10th and 1/5th of a mile. A circle whose diameter is 1024 feet has an area of about 0.0295 square miles, or 18.9 acres. For simplicity, I'll decide that, when I reach Colossal+++, I'll take up 16 acres.
HD | Size Category | Area |
1 | Medium | 43 square feet |
2-3 | Large | 173 square feet |
4 | Huge | 693 square feet |
5-7 | Gargantuan | 2773 square feet (1/16 acre) |
8-11 | Colossal | 1/4 acre |
12-15 | Colossal+ | 1 acres |
16-19 | Colossal++ | 4 acres |
20 | Colossal+++ | 16 acres |
The numbers for Colossal and smaller don't line up with the space that a character of that size is supposed to take up in combat, largely because those spaces were designed for tall creatures rather than sprawling plants. This won't matter as much once I get the Titanic Creature ability to treat myself as terrain.
16 acres is much smaller than
Pando, but bigger than
Thimmamma Marrimanu or
the Great Banyan. Unfortunately, the smallest US National Forest is 11,252 acres large, or about Colossal+++++++ (that's 7 +s).
Edit: I could use one of my Evolutionist teratomorphisms to grow another size category, which could get me to Colossal++++ (64 acres) at the expense of a very limited resource (I only get three of them).