Quote:....So it seems that I've specified everything correctly and GIMP is ignoring the image size when it comes time to print. ...
Is it ? Not according to your own screenshot. looked like 7" x 4.6992" print to me.
It is a shame you are using Gimp 3.0 There is a chance that MacOS has the Gimp Gutenprint plugin available for 2.10
It Is not mandatory for a graphics program to support printing, Krita for example, which has the philosophy that printing is more trouble than it is worth.
I am a somewhat biased when it comes to Canon. The printer driver for the one I had was terrible, everything centred around Windows. No support for linux. I used to print from a Win 7 VM
Edit: Maybe another go at explaining. The image size as set in Image -> Print Size is only effective when the image fits in the paper size, otherwise the ppi is adusted to fit.
The user can also adjust the ppi to set the image size, the printer does the scaling. This one straight from my camera, 4896 x 3672 pix and 180 resolution. Then printed 7" wide.