Whether the gallery you’ve created with Modula is a creative gallery or custom grid, you’ll notice a setting called thumbnail size. In short, this can be used to set the minimum width or height of the thumbnails in pixels.
When a gallery is set to creative gallery this option will as it suggests define the smallest possible size that a thumbnail will be displayed at. For a landscape image, this means that when the thumbnail size is set to 500 px, the image will be 500 px wide. For a portrait image, this means that when the thumbnail size is set to 500 px the image will be 500 px tall.
However, when a gallery is set to custom grid, images added to your new gallery will (by default) occupy a 2 x 2 square which can be observed when the helper grid is enabled.
In this case the thumbnail size setting allows you to choose how big one individual square from the custom grid should be. For example, if you were to set the thumbnail size to 300 px and resize images in the custom grid to be as large as 4 x 4 squares, the image will be of size 1200 x 1200 px in size.