Conduit for future sprinker system before sidewalk pour
The correct answer depends on a number of variables related to the size of the space and your future landscaping plans. If time permits, you should sketch out a landscaping plan and consult with an irrigation planner to ascertain in-ground requirements according to planned design and water supply.

If time does not permit, follow advice of @Platinum Goose and overbuild with 2" PVC, which would allow you to feed smaller pipes and/or wires through. I would put in at least two (one each end) and glue caps which could be cut off later, as suggested by @Jim Stewart.