Start Regedit and use the "Registry/Export Registry File" menu option to create a backup copy of your original registry. Use the following steps to set the background textures past a 1024x1024 resolution. In Regedit, Go to "HKEY_LOCAL_MACHINE", then select "Software". Select "PlayOnlineUS" and the "Square Enix" folder, then the "FinalFantasyXI" tab. You should see two "strings", "003" and "004". Double click to open them, and change the value for "003" to "800", then change the value for "004" to "800". You will notice the values to the right after you click "OK" will change to the corresponding resolution.

2048 x 2048: 800 and 800
1600 x 1600: 640 and 640
1280 x 1280: 500 and 500

Note: This code requires you to edit the Registry of Windows, be carefull when you make changes.