There are plenty of ways to go about changing the size of your thumbnail. You might want to check out https://community.auctiva.com/eve/forums/a/tpc/f/6881044121/m/8441035051, if that does not help try looking under this same thread Templates & Custom Templates for topics relating to image size, you should be able to find a couple of different solutions.
With regards to the way the images are showing up, are you using the custom template creator or are you choosing a template through your listing page and then defining your layout using the "Customize Layout page"?
Hope all of this has been useful.