Because Stable Diffusion and Automatic 1111 only accept pixel numbers dividable by 8, it can be a little confusing to match the image size to certain aspect ratios. I collected info about the most common aspect ratios in use, and with Excel formulas was able to pinpoint a lot of matching sizes which should work with SD 1.5 and A1111 (see table below). For SD XL, you may try to simply multiply the pixel numbers by 2. I haven't used XL much myself yet so not sure how well it would fit for this purpose - let me know in the comments if you think it would work, and I can make and upload an extra table with all image sizes doubled.
https://i.imgur.com/os3HsmB.png
(for PDF-version, see attachments to the right)
As for which aspect ratio to choose for an image with regard to composition and so on, that's another topic and a lot could be said about it (I'd suggest checking out some online photography guides). Some may decide based on how the image is going to be used. Or you may decide based on what kind of image you want to make (portrait, landscape). We typically use 1:1 or 2:3 a lot for Stable Diffusion on Civitai, but there are alternatives. For example, 4:3 or 3:4 is used in medium format photography, and 5:4 or 4:5 is common in large format photography (4:5 also happen to be common on Instagram...). So don't be afraid to try something new. And now you have a table for looking up a matching image size. :-)
Updated 8. june 2024 to version 2. Changes: Fixed a bug (16:7 was not sorted to the correct row in the table). Added ratios: 1:4, 1:3, 8:15, 8:11, 7:9, 11:14, 7:8, 8:7, 14:11, 9:7, 11:8, 15:8, 3:1 and 4:1. Added info about art canvases in "Areas of use".