You can pick tomatoes when they're green for fried green tomatoes or you can wait until the tomatoes turn red which means they're fully ripe.
Sometimes I'll pick some green tomatoes to have some nice fried green tomatoes and then I'll leave some tomatoes on the plant until they ripen to red tomatoes.
A fully ripe tomato will be really nice and red in color and firm feeling regardless of the size of the tomato and it may also have some yellow color around the stem.