Thank you for your inquiry. Yes the 17.5 is minimum height from the bottom of the glass to the top of the entire light fixture.

Just to clarify your answer, the top of the entire light fixture means the ceiling and not the top portion of the glass on the lighting fixture, correct?

Correct the ceiling down to the bottom of the glass. Thank you!

Can you adjust the length

The length on this fixture is adjustable anywhere from the 17.5 inches mentioned above and the longest length is 61.5 inches.

How exactly do the rods adjust in length?

Thank you for your inquiry. The wiring is fed through the rods. Rods are provided in a set of three 12", one 8" and one 4". Any excess wiring will be pulled through the top of the canopy, out of sight.