I believe you simply use the 'custom' choice at the bottom of the print properties box pop up when choosing a file to print, - then choose the very top radio button to input a "specific size" in the dialogue box that now appears and enter the size in the right hand entry boxes.I think you have to make sure you don't have "auto cropping ON" if your print was already sized to 12 x 36 in PS, (but not completely sure on that) .
David, what you say about using custom size is correct, however ....
To get the exact size you do normally need to have the crop scissors on and have to accept that part of the image may be cropped if the image and print aspect ratios do not match. For a PS sized image, crop scissors off should be ok.
For pre-seized (embedded size) images it is possible to use Custom-Use original/Embedded size with the Override box un-ticked.
Having said this, PS users coming to Qimage need to realise that "sizing" an image is totally unnecessary; Qimage has all the features needed to specify a print size with pre-sets (customisable) and various Custom methods.. This is in real linear dimensions; it's impossible for an image file to have linear dimensions, it really just has pixel dimensions.
The second very important thing to note is to never re-size (interpolate) the image, let Qimage do all that with it's superior methods automatically.
Terry