Changing Aspect Ratio in AE

Changing the Comp width and height in After Effects is similar to changing the Canvas Size in Photoshop.

If you've started animating, I would continue working in your 5175-by-2589 Comp and then nest that into a 16-by-9 Comp (you're using 1280-by-720 in your screen shot, but any 16-by-9 Comp size will work), scaling the nested to 5175-by-2589 Comp to fit to the containing Comp width (creating a letterboxed animation) or scaling the nested to 5175-by-2589 Comp to fit to the containing Comp height (creating a widescreen animation that crops the sides).  For example, nest the 5175-by-2589 Comp into a 1920-by-1080 or 1280-by-720 Comp and then choose Layer > Transform > Fit to Comp Width.  When you're ready to render, render the containing 16-by-9 Comp.

If you have not started animating, you could open a duplicate of your your 5175-by-2589 Photoshop document in Photoshop and make the desired Canvas Size change then Image Size change for animating in After Effects.  Be sure to add something like "for AE" to the filename of the duplicate document.  For example, in Photoshop change the Canvas size of the duplicate 5175-by-2589 document to 5175-by-2911, changing the frame aspect ratio from 2.0 to 1.78.  Adjust the layers as needed for the to fit the 1.78 aspect ratio.  Then change the Image Size to 3840-by-2160, 1920-by-1080, or 1280-by-720 (I'd lean toward the higher Image Size).  Save and import the resulting document into After Effects as a Composition.

Your original file is not 16:9. It is about 2:1, so something will get cropped when you put in a 16:9 comp.

It looks like you imported your PSD file as a composition. I can't tell if you chose Cropped Layers. Here are a few guidelines you should follow when creating a Photoshop or Illustrator file for video production,

1. For Photoshop, the file should be an even number of pixels high and wide - odd-numbered rows or columns are not allowed in any standard video formats used for distribution.
2. The frame size of your Photoshop file should be large enough so that no part of the image you will animate is scaled to more than 100%. That means if you want to fill the frame with one of the fish in your file that you want to fill half of a standard HD frame, then the fish should be at least 960 pixels wide in your original Photoshop file. Everything you are going to animate needs to be on a separate layer.
3. If you are creating an Illustrator file, snap to pixel and Pixel Preview should be turned on and the artboard should match the video's frame size in points or pixels.
4. The Color for PSD and AI files must be set to RGB.
5. There are no inches in a video or in web graphics, so the PPI setting in Photoshop is irrelevant. You only need to pay attention to the number of pixels

When you import an AI or PSD file as a comp retaining layer size, and the frame size or artboard does not match the frame size of the video you want to produce you need to add a null to the timeline, parent every layer to the null, then scale the null to get everything to fit inside the composition frame. 

Unless you are an expert in video formats and compression technology, you should always use the Composition Presets for frame size and frame rates. Anything that is non-standard will foul up any project you render to share on the web.

I hope this helps. It might be a good idea to spend some time in the Learn workspace and with the User Guide so you can get a better understanding of workflows and the UI.

Sounds like you're simply clicking on the presets. Nothing stops you from dialiung in your own numbers in the composition settings, though.

https://helpx.adobe.com/after-effects/using/composition-basics.html

Mylenium