The required size of the pipe and the distance it has to travel, plus how much water your house needs all go into deciding how big the pipe should be. If the pipe ends up too small, while the pressure will stay the same when nothings on, as you try to use more water than the pipe can handle, the flow can't keep up. Think soda straw verses fire hose...at a trickle, they're the same, but if you need it, you'll get LOTS more water out of the fire hose!
Without knowing what's going to be in the new house, it's hard to say what size you need. 1.5" supply pipe is bigger than an average house, but you may not be planning an average house. Are you measuring OD or ID? If OD, that could be much more common. IOW, need more info.
There are ways to minimize disruption to the landscaping depending on the soil, how deep the line needs to be and probably other factors. But, I doubt you can use it as a conduit for pex...it would end up being too small of a diameter for your needs.