A utility trailer no bigger than 5x8 lightly loaded would probably be the biggest I would go. I regularly use mine in that capacity, it's rated up to 1,000 pounds, but has hauled quite a bit more.
The major issue is that puts your brakes at the upper limit of their capacity, and you have little extra to handle sudden stops, etc. Just the same as a 80,000 semi on the highway - it's not going to haul you down very quickly at all. You have to be a lot more defensive in driving, stay out of dense traffic or be in the way. You have to react to things happening 2-3X further down the road.
Other than that, the mechanics of installing and using a hitch isn't rocket science, I'm using the 1 1/4" factory stock hitch and wiring that came on the 99. It's the same trailer I used behind a 4.0 Cherokee that had almost the same hp and much worse brakes, I considered the Forester an upgrade with all discs.
Balance the load on the trailer and keep at least 150 pounds on the tongue to ensure proper handling. Having the trailer actually neutral or pulling up on the hitch is extremely dangerous if it starts oscillating at speed. It will quickly increase to the point all control is lost and you will have an interesting time watching the world go by your windshield. Keep weight on the tongue. If the car starts nosing left and right you need to reduce speed immediately and if necessary, pull over. Get more load on the hitch and it should go away.