Expansion (Augmented) would be a two-size increase item - and the big difference otherwise is Expansion works on non-humanoids, where Enlarge Person does not.
If allowed, it would (for some duration) make me Gargantuan ... dramatically upping my combat and mass-transit ability.
Having discussed with real-life friends, it seems my strategy should be using that lengthy tail of mine to trip opponents. In general, it appears that natural weapons do not provoke AoOs from trips (we can't find anything that says they do) and as a Large quadruped, I'd have a +8 bonus on them. If you ruled my tail does have extended reach, that'll just assist in that way.