I think it all boils down to the available space, your intended use for the pool, decking and surrounding area, etc. We just installed a 14’x30’ fiberglass pool an I’d say it is on the smaller end of the medium size for a pool. I doesn’t feel too small for us, and it fills the available space. We would have went larger if we had more space, but this worked out just right for our use.
I also think it’s possible to confuse small - medium - large size with average size. The average pool size in my area varies, with more large pools on my particular street, whereas my pool is medium size. So my pool might be below average size for my street, but a street away the average pool is of the small size, so my pool is above average compared to that street!
So I would do what you’re doing, stake it out and lay down some rope or paint on the ground to sketch it out. Then stake out your decking and other items and see how it all feels. For fiberglass, remember that the measurement from the factory is for the outside edge of the pool shell, so the actual pool will be about 4”-6” less both width & length.
Also, for what it is worth, at least in my area, fiberglass isn’t required for salt water. Many folks in my area have concrete pools and salt water chlorine generators.