Rich,
The following is not a joke....
Something you need to do before you buy a jack, is let the air out of a tire. If you buy a jack that is 1/4" too tall...
Measure both under the axle and possibly the balance link (where Dutchman is showing) but only a floor jack with a big lift plate will like that. Remember, if a tire blows and shreds, you may be down on the rim and that can need another inch+/-.
Bottle jacks come in all kind of sizes. If you don't know the axle load, at least get the loaded trailer weighed so you have some idea.
Assuming you have chocks in your kit, you will still need a wrench. Buy a good Cross wrench (aka 4-way) and make sure it has the size for the trailer nuts. (Some these days have metric ends.)
If you aren't carrying a spare, don't bother with any of this. The road service guy should have all that.
If you are going to carry this kit, loosen (and re-tighten) a nut or two at home, and try jacking up from a flat tire too. You don't need to find out if the kit doesn't work at the side of the road, in the dark, when it is raining and cold.
Matt