Bundler will by default set the width of the product images in the widget to the optimal size based on the available space. However, if you want to modify the size of product images, you can do so with a custom CSS selector which you add to General settings -> Custom CSS.
In order to do that, we suggest to use the .bndlr-product class selector and set the max-width property to the desired value.
Contact us if you need any specific help with this.