If you're stuck on something like this in Biology, I would really suggest revising ASAP. It gets a lot harder.
Using the term water potential explain how the plant cell size will increase in water
I’m assuming you mean pure water which will have a water potential of 0KPa?
In that case: The plant cell will be hypertonic to the water, so water will move down a water potential gradient by osmosis into the plant cell. This causes the cytoplasm and cell contents of the plant cell to expand and push against the plant cell wall, the plant cell wall will push back with an equal and opposite force and so the plant cell will become turgid.