Bellows Control Panel > Main Configuration > Images
When you upload an image to WordPress, WordPress automatically generates multiple sizes of that image. WordPress core, themes, plugins, and your own custom code, can all define the sizes that should be generated for each image.
When you add an image to the menu, you want to use the smallest image that fits properly, to minimize bandwidth and maximize the load speed of your site for your visitors. While it’s generally best to upload images at exactly the size you want to use them, you can choose which Image Size you want to use for the images in your menu via this setting.
By default, the images will be displayed at their full size (original upload size), unless you change this setting.