Will the size of a onion cell change when kept in hypertonic solution?
The cells will become smaller.
If the solute concentration is higher in the environment (outside of the cell) the cell will become smaller because it will lose water due to osmosis.
This video discusses the plasmolysis (loss of water) that occurs in red onion cells when they are placed into a concentrated salt water solution.
This second video is footage from a slide of red onion cells originally made with distilled water. Part way through the video a drop of salt water is added to the edge of the cover slip. The salt eventually diffuses under the cover slip and causes plasmolysis to occur in the onion cells.
Hope this helps!