Stupid question I guess, but I'm unsure and I don't have a manual.
How much water should be in expansion tank when engine is hot and when cold?
Plus where can I find a Haynes type manual??

 

Assuming it's a newage. The top tank should be full, the one next to the rad has a min / max mark. The are no Haynes manuals for the impreza, you can find workshop manuals in the technical area on here.

 

Check the coolant level on outside of the reservoir by battery when cold. If its low fill to full mark.
When the engine is hot the Coolant will expand over the full mark and return to full when stone cold.

If it returns to lest than full when cold there maybe a leak or using coolant.
If it stays over full mark when cold air maybe getting into system.