Where to tie in for auxiliary water line?

I plan to run water to an outbuilding I constructed which has raised a few questions. I'm using 3/4" or 1/2" PEX and the distance is approximately 40' to the building. The building will have a full bath (with water heater) and an extra sink in the shop area. So...should I run 1/2" PEX to the building or 3/4"? (does it matter?). And perhaps most importantly, how best would I attach to the source? The supply line coming into my house is 3/4" copper pipe. After the pressure regulator, it is 1/2" copper throughout the house. I wondered if I should simply "T" off from a 1/2" cold water line somewhere and just run it directly to the bulding. Or should I install a 3/4" 'T' on the incoming line (before the pressure regulator) and install a SECOND pressure regulator there, just for the outbuilding? Wasn't sure if it was OK to have two pressure regulators. Lots of questions here; any help would be greatly appreciated!