Do it yourself: If you want to write your own version of this macro for your favourite maximum size, these are the steps: Layer > Unlock
Layer > Rasterize and Trim
Filter > Distort > Equations Enter the following in the x and y fields, substituting your own maximum size, then Apply
   (x/MAXSIZE)*max(h,w)
   (y/MAXSIZE)*max(h,w)
Document > Clip Canvas
Layer > Rasterize
Select > Alpha Range > Select Partially Transparent
Edit > Fill> Inpainting
Select > Deselect I thank @carl123 for the last four steps. EDIT: These macros have been edited to add the first two steps. Unlocking is necessary, but I had not previously included it in the Macro. The Rasterize step is necessary if the image has been cropped previously. EDIT 2: I have amended step 2 to use Rasterize and Trim (rather than just Rasterize). This is new in 1.7.