My laptop's resolution is 1920x1080 but i usually have the windows scale size at 125% and to play ffxi on 1920x1080 i have to rescale it to 100% every single time.

What resolution should I set the game at to have it fit my screen at 125%?

Instead of doing the workaround you're thinking of, try a different one:
Right click your Ashita.exe-> Properties-> Compatibility-> "Override high DPI scaling behavior. Scaling performed by: ..." and set the option to Application.

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