Gallery thumbnail sizes can be changed using the Theme Editor. You can change the thumbnail sizes for a specific gallery, gallery type or for all of your galleries. Once you are in the Theme Editor you can follow these steps to change your thumbnail sizes.
Navigate the viewer screen to a page of thumbnail images.
In the Theme Editor click on Customize Image Grids.
Click on Image Size and choose one of the options there.
Adjust your Spacing, Borders and Custom Roundness Level.
Click Save at the top or Save As if you wish to save this as a new theme.
