Gallery: inset/lightbox/grid:strips - gallery is too tall when only 1 image is uploaded

my image is too tall when i only have one image in a gallery. how can i set a fixed height to match other pages that have 4+ images and prevent the block from automatically sizing to the image size?

this is how i'd like the image to look even when there is only one image. centered, preferably. 

https://www.koblecreative.com/work/brasada

thanks