I decided to have some canvases printed by a local firm. They usually stretch them also , but will discount by over 50% if they do the print only.
I send a print in, sized 19x15 in (includes 1.5 in per side for gallery wrap, so final canvas should be 16x12).
What I got back was a print 19.5 x 16 in. I had reflected the image on all 4 sides - the image printed was missing one of the added sides and was enlarged compared to the original.
The owner claims that the machine (epson) can't cope with cropped photos & gives unpredictable results unless it has the original dimensions.
I am inclined to think that he doesn't really know how his machine works ( he only produces prints 20x20 & 20x30) and prints them a lot larger than necessary & then adjusts when stretching.
I asked him for the ppi & printer profile & he didn't know what they were!