Crop, resize, and print small images on 4x6

[machVA]

Hi. I am looking to something seemingly basic but it is eluding me.

My goal is to print pictures (using a printer at a drug store, Walmart, or through Google Photos print ordering service) that are of a specific size and aspect ratio. The resulting print will be on 4x6 or 8x10, but will not fill the entire sheet. Rather, it will be of a specific smaller size. The end picture will be trimmed to fit in a collage picture frame that uses non-standard sizes.

Here's the workflow I have been trying, with some example numbers:
1. Starting with a JPG image (~3200 x 2450 x 24 BPP image size, 528KB file size) in IrfanView.
2. Crop the image to the aspect ratio needed (1:1). End result is an image of 2105 x 2105 pixels.
3. Resize (Ctrl+R) to the needed image size (3.55" x 3.55"). End result is an image of 256 x 256 pixels.
4. Save the image, using the "JPG Save Quality" set to 100. End result has a file size of 67KB.

There are two problems with this:
1. The saved image is of a very low number of pixels. Maybe there is no way around this?
2. The printing tool I am trying to use (drug store, Walmart, Google Photos print ordering service, etc) wants to stretch the image to fill the 4x6 size, as well as complaining of the low resolution.

It seems that I need to create an image in IrfanView that is a 4x6 image of whitespace, with my desired image centered on the 4x6 image.

Can you advise on a good approach to achieve my goal?

Thank you!

[Bhikkhu Pesala]

Change the DPI, not the image. 300 DPI is recommended for printing, so 3.55" should be 1,065 x 1,065 pixels. Then add a white border (Canvas Size) to make the size 1,800 x 1,200 pixels (6"x4")