What's the smallest general-use tractor I can get?
The problem is, there are infinitely many jobs that a tractor can fill, if you look creatively at how to use that tractor. A small tractor can do some of those jobs. At some point, you will find that only a larger tractor can solve some more complex job. Of course, a larger tractor will take up more space to store. It will maneuver less easily in tight spaces. A heavier tractor may compact your soil more in your lawn. There are always tradeoffs to make.

It depends on what you are looking for. Large manufacturers of tractors, Kubota, for example, or John Deere, offer a huge line of tractors at all levels of size. Both of these standard lines of tractors offer many good choices. The more money that you spend, the more capability achieved.

For example, I can move stone and gravel with the bucket on my tractor, but only a few cubic feet of it at a time. Over the last few years, I've moved about 30 yards of stone and gravel. However, there is one rock I want to move, but it is about 200 pounds too heavy to budge. The hydraulics on my tractor simply won't lift it, but a tractor with the capability to move that rock simply won't fit behind my car into the back of my garage.

As an alternative to the great tractors offered by the mainstream vendors, consider something somewhat less conventional. There are small tractors that can agilely fulfill many purposes, capable of mowing a lawn of any size, yet move dirt, gravel, large stone, dig a ditch, run a trench, move a boat trailer around, pick up a pallet, grind out a stump, pick up a pile of brush with a grapple, etc., and do all of these things with only a few seconds in between tasks.

First, I'll point out Ventrac. These tractors are small enough to mow a lawn, but they offer something that can mow your lawn, but in about a minute's time can swap the mower deck for a bucket that can move dirt, stone mulch, etc.

Similar to Ventrac, there are Steiner tractors, another interesting, very capable line of small tractors for the homewoner.

Next, you might look at Powertrac. Powertrac is even more capable in some respects, with dozens of implements you can attach in a matter of seconds. Look here for some examples of a PT in action.

And, there are even relatively small walk behind (two wheel) tractors. Take a look at BCS.

A very neat line of tractors are the various Magnatrac models. For the grown up kid who wants to move some serious dirt, this is the trick.