Symptom

When I add a large image (for example, 2000px width) to my page and I try to change its width using the custom image size UI, it won't let me enter a pixel size larger than 900px (for example, 1200px).

Workaround

Resize the image to the desired size in an image manipulation program. Insert the image and do not attempt to modify the size in Confluence, and it will display in the desired size. The image will only have an upper bound of 900px when resized within the Confluence editor.