Size does matter (for photos on canvas)
If you print a photo in a standard photo size, the number of pixels does not really
matters. But when it comes to printing photos on canvas, it is another thing.
To make big canvas photos, you need photos with high resolution (meaning a lot of
pixels).
If you only got a 2 megapixel camera, you can still get a good small-size canvas
photo, but to get one of the bigger ones, you need a newer camera with much more
pixels. With an 8 megapixels camera, you can have canvas photos in even the largest
sizes.
You can find a chart with the recommended minimum sizes on for example Foto.com.

Find the minimum recommended photo size on the Foto.com website
It is possible to print your photos on canvas even if the resolution is lower than
the minimum recommended, but there is a risk that the will look blurry.
On the other hand if you are only looking at the canvas from a distance, it does
not matter that much.
Remember also to check, that the format is the right for the canvas (read more about
the format in the article
Choose the right format for your canvas photos).