I recently purchased a new m7040sud. It's the basic model of this tractor. Uses the same loader (w/o the third function) and it has a basic gear transmission with a mechanical shuttle. I have the 4 wheel drive version. It's a great tractor at a good price.
I live on 40 acres. The 7040 replaced an old Massey Harris MF50 (~30). Even for 40 acres, I wanted something bigger. While that tractor had good power for a 30hp tractor, it didn't really push snow that well and when it came to pulling heavier things (large rocks, 2-3000lbs) it would lose traction. The 7040 was a great upgrade.
For 300+ acres, I don't think I'd want a tractor much smaller than that, if smaller at all. I don't think you'd regret going bigger, if your budget allows. A piece of advice I've heard on here several times, though, is it doesn't matter a great deal how big the tractor is, it'll just take a little longer for most things the smaller the tractor is.