The growth thing sounds like it's best implemented by a racial feat, with prerequisites of the PC reaching a certain age (and perhaps spending X amount of time rooted in the ground, sort of a hibernating or nearly chrysalis stage they have to go through to undergo a major growth spurt?)
As far as the actual benefits of growth, that's a thorny (heh) question. Just eyeballing it, I'd give them the permanent benefits of an Enlarge spell (which is veeery roughly equivalent to a +2 Strength ASI) - the 10x10 footprint i see as a bit of a double-edged sword. Then, to make it a bit more interesting, I'd probably add +1 Con and a speed penalty. My instinct is a 10ft penalty, though that'd probably be too harsh if they have a base 25ft movement, so 5 might be the way to go. I just like the idea of the cactus-people becoming ponderous as they grow.
Not sure I'd allow PC cactuspeople to grow above Large. If nothing else, how many campaigns would last the many in-world years that'd be required for that to happen?
