Image Size

Hi @fabioweb.
Thank you for reaching out.
In the widget settings, in the section Thumbnail Settings you will find a dropdown with the label Size of thumbnail. There you can select the size you want to use for your thumbnails.
This list shows all registered images sizes. If you do not find your desired size you need to add it by using the function add_image_size().
For the global thumbnail size you can also set the value in Settings > Media > Thumbnail size. Please make sure this does not effect other layout parts of your theme.
Beside this you need to generate the thumbnails after changing or adding an image size. There are a lot of cool plugins on ww.wp.xz.cn doing this.
I hope that helps.

Yes, this settings will not create the image. It is just the display size.
So, you can choose an image with 150x150px but show it as 75x75px.
If you want to load the 75x75px image you need to add the image size support, if oyur theme does not provided it. WordPress will than create the desired file when you upload the image.