I am running a PEX line underground from my main house to garage. There was an existing galvanized line here but is busted halfway probably due to freezing before I purchased the home. I am going to run PEX underground from the galvanized line as is exits the back of the house and my question is how would the pressure be affected if I used 3/4" PEX instead of 1/2"? I know in an HVAC scenario you always reduce the trunk-line the further it gets from the supply to ensure equal air distribution and unsure if this would also apply to a water line. The initial connection is about 40' from the 3/4" main as it enters the front opposite corner of my home and I would be running about 120' of PEX. I hope to install a toilet and shower back there one day but for now it's just for a hose connection.

i'd run 3/4 pex..
how you connect to the galvanized depend on the condition of the outside of the pipe. to be safe i would use a compression dresser coupling to attach to the old pipe. use a nipple and a pex adapter on the other side of the dresser.
our sponser even has them as male by compression. (take your pick)
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