I don't print regularly so I could also be forgetting something .I did open the pdf manual but using the QImage 4 x 6 template and the printer page size results in a slightly less than 4 x 6 and the warning.
Rose
Rose,
There are two parts to your question and therefore two answers.
1) No need to, and what's more, a reduction in quality when you interpolate an image more than once.
So put the image into Qimage, and crop the image or the print, and Qimage will take care of the interpolation needed to make the very best print at the size you ask for.
2) Your printer, most all printers, have, what is called a printable area of a paper. In the case of a 4 x 6 on a 3800 Epson, is 3.766 x 5.766.
That will be the maximum size print on that paper using normal settings.
If you want full 4 x 6 prints, simply click on Borderless in the driver, and selecting 4 x 6 paper, tell Qimage to use the size called FIT TO PAGE, and print.