Ive been asked to create a website mock-up using Photoshop, but I'm not sure what size to use for my canvas.
Please could you help me about that? or give me an advice?
This is a hard question as mostly websites are adaptive. From my experience I would say the height doesn't really matter if you allow scrolling, otherways I would take 700px for height. The width should be no more then 1200px.