I used fresh roma tomatoes for my soup and they were perfect. Larger tomatoes will not lose as much moisture so consider adding less vegetable stock if you're using bigger tomatoes.
To make this recipe with canned tomatoes, used a similar amount of canned tomatoes(approximately 60-70 oz canned tomatoes), drain the tomatoes reserving the liquid for later and spread the drained tomatoes on the baking sheet and proceed with the recipe as directed, adding back the juice from the canned tomatoes instead of vegetable stock or water.
Since roasting the tomatoes and garlic doesn't really add much smokiness I think a dash of liquid smoke would be delicious in this to enhance the roasted flavors!