Advice on how to fix an STL file

I'm hoping this is the best place to post for this but I recently picked up a file for some oval miniature bases that seems to be just a bit off from the standard ones on the minis I got. The STL file seems longer than the 42x75 base I have.

Initially, I could make them toppers to the base the minis came with but the oval shape is not the same as the standard bases. So I want to edit the file to match that shape but I am not sure how to start in 3D viewer. Any advice is appreciated.

This base seems to be a longer oval shape than what I was using.

https://preview.redd.it/0r4m7qr3jmjd1.png?width=1536&format=png&auto=webp&s=0ea5fccd33e806d337e445081f1ce2ba2b3316ea