Edit: misunderstood the question the first time through.
So you want an unlimited use Command word ring of Enlarge Person?
By the guidelines, that's, what, Spell Level 1 x caster level 1 x 2,000 gp for a continuous or command word item, x2 for a duration of minutes/level ... it's a transformation, so the affinity is for cloak/cape/mantle; as a ring, it's thus off-slot and needs the x1.5; so 2000 gp for spell, x2 for duration in minutes/level, x1.5 for off-slot -> 2,000 x 2.5 -> 5,000 gp, 4,000 gp for a cape/cloak/or mantle (arguments could be made for robe, shirt, bracers, gauntlets, or belt, as well) - maybe.... also maybe push it up to caster level 3, which would triple the price to 12,000 for on-slot, 15,000 for off-slot (or CL 5, for 20,000/25,000).
Comparing to similar items... well, let's see.... that's +2 strength (4,000 gp right there), but -2 Dex (what's that worth, I wonder? -4,000?), -1 AC (in addtion to the loss of AC due to dex; what's that worth?), -1 to hit (although the Str jump cancels this - exactly - for a meleer), extra reach, and extra damage. The damage increase due to going up a size category is roughly +50% (not counting str boost) (see table below)
What's that worth? Well, the Monk's belt increases a monk's unarmed strike damage by a similar amount, but also increases AC, at 13,000. If you assume that the monk's AC boost corresponds to the reach benefit of the size increase of the Ring of Growth, that the AC loss from the Ring of Growth doesn't much matter, and that the Str increase balances the Dex decrease, then you are justified at pricing the Ring of Growth the same as a Monk's belt (13,000 gp, by SRD).
Pre and post from the Improving Monsters section of the SRD; averages calculated by me
Code:
Pre Post Avg Pre Avg Post % Avg Change
1d2 1d3 1.5 2 +33%
1d3 1d4 2 2.5 +25%
1d4 1d6 2.5 3.5 +40%
1d6 1d8 3.5 4.5 +29%
1d8 2d6 4.5 7 +56%
1d10 2d8 5.5 9 +63%
2d6 3d6 7 10.5 +50%
2d8 3d8 9 13.5 +50%