Ok, so lets say I’ve finished modelling my 3d model inside ZBrush and I want to render it to get .psd file to edit in photoshop. However, my canvas size is only somewhere around 1530 x 960. So when I render it, the pic really doesn’t have the resolution I was looking for if I want to make it bigger (a3 size lets say). I know that I should have started with a larger canvas size to begin with with AAHalf in mind but I didn’t so…basically did I just waste all my time making my model only to have it come out as a low res image after rendering. I thought that the 3d model resolution wasn’t dependent on canvas size…Is there any way to combat this and get a better high res image?? Any help will be appreciated bcoz I really don’t understand this rendering business at all. 
Go into the Document menu, turn Pro off and make the document whatever size you want. Using the Zoom button on the right side shelf (or Zoom menu), zoom out until you can see the whole document. You will see the document edges. Draw your tool onto the canvas so it fits into the document. Render however you want then export the document.
Phew…you don’t know how grateful I am. Thankyou 
Your welcome! I don’t understand why people do this AAHalf stuff. when you can get exactly what you want this way, and it’s easy.
edit: don’t do sculpting work in huge documents though. Just do it when you want to render a huge image.