Script for resize to 1000x1000px and save as .jpg in 300dpi

Then all you need to do is use one of the image processors you just need to use the resize option and set width and height to 1000.  If you use the Image Processor Pro  Plug-in script you can also specify the interpolation method to use, No Action is required.

The Fit Image command will fit the longest edge to 1000px, however if you require a square output of 1000x1000px from a rectangular original, then you will need to add two steps to your action to resize both canvas edges to 1000px. This will “letterbox” or “pillarbox” the original rectangular content into a square. In this example, the current colour picker background colour is used to letterbox, however you could preset white, black etc into the action. A final step to resize the image with resampling OFF (no pixels added or removed) to 300ppi/118.11pcm is also added to set the pixels per inch metadata value (which may or may not be retained depending on the save method).

The save to JPEG could be included in the action as a 5th step, or as JJ mentions the file format output could be handled via Image Processor/Image Processor Pro and then you don’t need the JPEG save recorded into the action (which would then require the use of “override” options in the batch command).

This of course presumes that you are OK with having the images “letterboxed” – otherwise you would need them to be cropped to a square first. This can be automated, however you would likely wish to decide which pixels are included rather than have this automated.

Are you image square to begin with.  If not you if you do not use restrain in images size you will be able to set 1000px by 1000px in Image size and all you none square images will be distorted into a 1000px by 1000px square image.  I do not think this is actually what you really want to do.  If you want 1000px by 1000px image files you have two other choices. Crop you image to square then resize to 1000px by 1000px or, Add extra canvas to make your  resized image square 1000px by 1000px.  Images that were not square to begin with will have borders on the width or height.

Which of the three do you want to do. The is a Forth option the results will be your image will fit within 1000px by 1000px however none square imags will be smaller the height or width will be less than 1000px.

In your screen capture Imags Size Constrain is on Width and Height are linked setting one will change the  other to constrain the resize to the current aspect ratio.

To do option 4 Fit Image all you need do is use photoshop menu File>Scripts>Image Processor....

Or Frome Bridge use menu Tools>Photoshop>Images Precessor....