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?
I'd simply suggest to stick with some grid framework, eg. 960.gs: http://960.gs/ You'll get a PSD grid to fit your design into and also CSS framework which you can use later for website coding.
For a more up to date grid framework I suggest http://unsemantic.com/ since it allows for a responsive grid.
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.