Image and Canvas sizing

[nikoi]

Hello,

This is my first post here so please go easy guy's.

Right, I'm in the process of create a series of digitally manipulated images in CS6, but I'm still not too sure on the final print size.
What would be the best approach in sizing up a canvas which I can later change to a larger canvas/print size.

The final images will be pretty large maybe around 40 x 40 inches. So my question, is there a way of working on a smaller canvas size
which can later be changed to a larger size ready for print, with jeopardising quality?

I just want my work flow to run a little smoother as working on a large canvas size like this really slows down my mac's performance.

Any help would be greatly appreciated!

thanks,

Nikoi