I would like to pre run underground conduit for a future above ground SPA. Load is 50-60 amps and location is about 100 feet from the main panel. Conduit would eventually be used to feed a sub panel at the spa location.

What size wire would be suggested and then what size PVC conduit could be used to pre-plumb this run?

Is this a 120v or 240v installation, or does it require both?
As far as wire size, it'll be either #6 or #8, depending on what the spa actuallyrequires. 1" will make it easy for either, assuming you are pulling 4 wires. The tub MFG. will tell you what size circuit to provide. (50 amps seems a bit high to me)