size of water supply line

my home sits 400 feet from the water meter. the supply line is 3/4 inch pvc. from the meter there is a 25 foot drop to the bottom of the hill then it rises 25 foot to the top of the next hill. what size should my supply line be? i am thinking of using a 2 inch line to within 25 feet of the house then reducing to 3/4 inch. will this increase my gpm? this is a old house built in the sixties i cannot use the water faucet at the bottom of the hill if we are using water in the house due to the loss of volume/ presure. the main reason for this questioning is that i plan on installing an irrigation system but when i checked my water flow it was only 6-7 gallons per minute i am trying to get around 20 gpm. tahnks