I have a tomato sauce recipe that calls for 1 can (28oz) of diced tomatoes (including the liquid). How can I replace the can with fresh tomatoes? I'm not sure if there is a particular type of tomato I should use or how many of them to use. Also, what can I do to replace the liquid? 
If you are really set on doing this, just use the same amount by weight or volume, preferably peeling the tomatoes first. (Cut an x in the bottom, dip in boiling water for 10 seconds, pull off the peel). You'll have plenty of liquid coming out of the tomato, don't worry about that. But honestly, I don't recommend doing this. Fresh tomatoes that are any good are so good uncooked that it is a waste, and fresh tomatoes that aren't good won't get better by cooking them. Even Mario Batali says you should use canned tomatoes for cooked sauces. 