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Try creating an action:

1. File > Automate > Fit Image (and set 1050 for width and 900 for height)

2. Image > Canvas Size (uncheck "relative" and set values 900 for both height and width)

Should work but it will crop sides since you have rectangle images and want them to be square.

if you don't mind stretching images just use:

1. Image > Image size (check "resample", unlink "chain" and set values 900 in pixels)

Hope that helps