How do I increase the displayed size of an image when using Lightbox in a Gallery

Our state natural history society wants to be able to share graphic data for each of approximately 170 species of dragonfly, illustrating which counties they can be found in, and what time of year they are active.   These graphics are batch generated from a SQLite database and currently take the form of PNGs that are consistently 700 by 1250 pixels.

Graphics will be regenerated at least once a year, and we want the simplest possible way of updating the online graphics.  The set of images in a Gallery is extremely easy to batch update, thumbnail layout is configurable, and the Lightbox option makes them individually viewable at a more convenient size.  This first experiment is pretty close to what we need.   

But the Lightbox size is undesirably small.  How do I allow it to display images at full resolution  (like it does below)?

I'm thinking that answer involves custom CSS, but I'm new with Squarespace so I'd also appreciate some guidance on how to associate the new config into a Gallery block.

And if this not the easiest way to meet ourrequirements, I'm open to other suggestions.