You can customize how the recommendation looks like in the Design Settings on the admin page with the following steps:
On your Admin Page, select Upsell Recommendations:
On the Upsell Recommendation Admin page, select Design Settings:
On the Design Settings Page, go to Style Tab, adjust the Product image settings while it seems fit, and click on the Save button:
Note: You might also want to adjust how many products you want to display at the same time. It has a role in the image size, since you have a limited place to display your products. You might want to display 2 products with large images or 4 with smaller ones. You can play with the settings to make it just right.
But how you can do this?
You can do this on the Content tab, on the Design Settings page: Adjust the “Max Products per slide” value, then hit Save. Make sure that You select enough products for recommendation in the “Number of products” section.
If you need more flexibility, you can create a custom design in your template using Custom CSS.