Primarily, thumbnail size is controlled through the Target row height and Row height max deviation (+-) settings. Justified Image Grid calculates the optimal size for your thumbnail within the range you set. It respects the original aspect ratio and only crops if the range you set is not high enough to make the justified layout mathematically possible.
The aspect ratio can be the original, or overridden by your custom, fixed Thumbnail aspect ratio value. It's not possible to directly set a fixed thumbnail size in pixels, but the aspect ratio control is the next best thing.
The Randomize thumbnail width is available to spice up a grid of similar aspect ratio photos.
Learn more about the plugin: