Postal mail is printed, not electronic. Social media and web images are fine at 72dpi or 96dpi, (dots per inch) and there's no reason to make them any bigger because that's all the highest resolution monitor is going to display.

But print needs denser images. Minimum should be no less than 200dpi and no more than 300 dpi -- in the size you want. Meaning, you can't post a 2 inch x 3 inch image (which looks fine on your screen) and expect it to look fantastic when printed at 6 inches by 9 inches. Your 300dpi image will become 100dpi and appear pixelated.

You also want to allow for the image to bleed (extend beyond the boundary edge), so best to have additional pixels to the image.
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6"x9" = 1871x2771px (keep your message inside 1800x2700)
A5 = 1819x2551px (keep your message inside 1748x2490)

Be sure to save your image at 300dpi.

Be sure to save your image as a jpeg (.jpg)

Don't use png

Here a sample guidelines to use:

6x9:

A5: