Tankless water line????

Hello, I am in the process of installing a new tankless water heater which needs 3/4" supply line. If I tie into a 1/2" line does the distance from the tee matter?
I was told that I need 3/4" for the flow rate.
I also have 135psi coming into the house which is in the process of getting a reducer but just wondering what will happen to the pressure when I go from 1/2" to 3/4" line????
Thanks!!!