How to check that uploaded image will not be blurred?

Hi
I've been reading through threads about getting pictures printed to canvas.
Updone seem to have the best rates.

I can't figure out though whether a picture would end up being blurred/pixelated, depending on the final canvas size (eg 24x24).
How can I enlarge my pictures on the computer screen to a particular size, to see if blurring/pixelation occurs or not?

I use Irfanview software for photos, but when I enlarge the pictures on it, I am not sure of the actual inches/cm they have been enlarged to, if that makes sense?

Any help appreciated.

Thanks

Nicola

Register with photobox.ie and you can upload photos and try them out for canvas. It will tell you if the quality is sufficient with a little happy or sad face. I disregarded their advice once and got a badly pixelated photo in return!