First time homebuyer

My boyfriend and I are trying to purchase our first home. How can we find out what size loan we can get? We have a substantial down payment and are planning to pay off all our debt immediately. We are trying to figure out just how large a loan we can get so that we can factor that plus the down payment and come up with a price we can afford. His credit is good (almost 700) and mine is so so (about 650). We just want to know so that we can find out if we might need to borrow money from our parents and if so, how much.