An American twin is like 39"x75" so I would definitely go a little longer.

That is beautiful! I agree with Jackie - I would make it longer. It would be easy to make the outside borders wider to accomplish that goal.

I'd make it longer if you are thinking of using it on a bed.

The pattern has already been "made longer" by the addition of the two panels above and below the main panel. You could cut the main panel "taller" or, if that's not possible, you could cut the two other panels "taller." The amount you increase the height of these panels (and thereby the quilt) should equal the dimension of two of your on-point squares. This is to keep the alternating colors of your squares in order. I hope this makes sense.

In researching this lightning bug glow in the dark pattern and fabric and wanting to enlarge it the easiest way possible, I would make the last blue border wider and the ending border wider. Or use the very dark blue fabric that's around the panel as the first border, then the blue border and the last border. You can then determine the size of quilt you want to end up with without having to mess with the measurements of the squares. Adding 5 inches on each side would make it 63X80 which is something she can use for a long time on any bed. This site might be useful:

https://www.quiltingdaily.com/suggested-quilt-measurements-for-common-standard-bed-sizes/