Listing Carousel Thumbnails inconsistent

Hi!

My website’s issues are endless xD Okay, so if you have a look at my homepage [https://www.thedollbazaar.com], I have recent listings as a carousel under my search box, which have thumbnails. Now they are all different sizes, the big pictures actually stretched the whole carousel, which I do not want. I would like to have the carousel one size and then the larger images/thumbnails constrained inside it, instead of having each box a different size.
OR to only allow my users to upload a specific maximum image size so that it does not distort the carousel, both ways, I have no clue how to do.

Also, the image in the slider in this page:
https://www.thedollbazaar.com/listing/by-testbed/
… is stretched out a lot, I would like for that to be constrained as well!