I would not run less than a 1". When I ran a new 1" feeder for my well house I also added a 3/4" and pulled in cat 6. I have never used it, but the PVC conduit was cheap at the time and I already had a trench. I can always use it for something else.
Gogar's suggestion to install wire as you go could be a pain. I have only pulled after installing. While my usual method is to just run a snake, but they do make sponges that connect tie onto drag line and blow or suck it through with a vacuum. Using a wet dry vacuum with a plastic bag tied to drag line has worked when the Greenlee was not on the job. Wire Ease Lube is highly recommended in PVC.
In NYC we always pulled in a drag line with the wires. It comes in handy if any additional wires needed to be pulled in later.
I'm not a fan of direct burial cable.
