About the heirloom tomato. Heirloom just means that the tomato is a non-hybrid cultivar. Heirlooms tend to have more complex flavors than hybrid tomatoes. What you want here is a beefsteak type tomato, a big one that's densely packed with lots of seed compartments. And not too much loose gel. Each thick slice should hold together easily in a big slab. Kind of like a steak.
About the bread. I like bread with a little whole grain in it. Maybe some seeds. It gives the sandwich a bit more chew. But a round loaf of crusty white bread works too. Just cut a big slice from the center, then cut the slice in half for your two pieces of bread. Or, if you have a smaller batard (oval loaf), cut two slices from the center of that. That's what's shown in the photo here. Note that the loaf in the photo was a little smaller than I usually use. That's why there's only a couple slices of mozz in that sammie. Adjust the amount of filling to match the size of your bread.
About the mozzarella. You want the fresh round mozz the size of a baseball. It'll be about 8 ounces. Just cut a few slices from the center of the ball. If you go for smoked mozzarella, that usually comes in a brick. Slice about 4 ounces of that into thin (1/8 inch thick) slices.