Using foil on your pan will make clean-up easier, and parchment paper will prevent your tomatoes from sticking. Be sure to use a rimmed baking sheet to catch any juices.
If the end-pieces of your tomatoes end up forming bowls, liquid will accumulate in them during the cooking process and cause them to steam rather than roast. Poking a few holes in the bottom eliminates this problem.
These tomatoes are delicious sprinkled with chopped fresh basil. Parsley is tasty as well, or you can use a combo of basil/parsley/oregano/thyme.
The first time you make these, and particularly if you are using smaller tomatoes, really watch the cooking time...especially towards the end, as they can go from done to burnt quickly.
This amount of olive oil and balsamic vinegar is appropriate for a 18x13x1" sheet pan full of tomatoes, regardless of the size/types of tomatoes that you start with. If you use less tomatoes, be sure to cut back on the oil, vinegar, and seasonings.