How do you resize a page to match another page after combine?

I was new to home scanning so I didn't do them at the same time like I do now so it seems one saved smaller or bigger than the other. They are in 2 different PDFs. Combining to make it easy to just scroll instead of coming out of another file to see the other. How do I make the bigger one smaller or the smaller one bigger? Is it too late? Is it in raster instead of vector?
It's the front and back of the same paper.
