SQ,
On the Qimage side of the equation, Qimage will always show you exactly the size that is being sent to the printer so if you look in the queue or on the preview page and you see 12.34 x 45.67: that's exactly the size that is being sent to the driver because Qimage is showing you the width and height of the actual data in the queue ready to go to the driver. In other words: it can't be wrong. If you get anything but that exact size, you know the shrinkage (or expansion, as is sometimes the case) is being caused by a factor outside of Qimage: something beyond Qimage's control.
I guess you've already determined that the issue is shrinkage (trying hard not to reference a Seinfeld episode)
but a good way to definitively determine if that is what is occurring is to measure the paper width before and after printing. Don't assume the paper is exactly what the manufacturer says either: actually measure the width before it goes through the printer. Then measure it afterward. The after number will be smaller if it's canvas shrinkage.
Also keep in mind that a given canvas will rarely shrink by the same amount. The amount of ink used (depending on the photo's subject matter) and even the colors used in the print will alter the final shrinkage amount. You might be able to get a decent average but you'll never get it exact.
Mike