[Q][L] lengths of lights/fixtures

I know that, supposedly, that "there are no stupid questions."  I 
don't think that the following questions break this rule, but they 
_do_ make me wonder...
	I and my freshwater plants want more light intensity.  I am perusing 
various catalogs in search of a double-row light fixture.  I have a 55 
gallon tank that is essentially split in half with respect to 
lighting; each side has a 23" (2'?) fixture.  Each of these fixtures 
holds a 17" (18"?) fluorescent bulb.  This is where my confusion 
originates.  How exactly are bulbs and fixtures measured?  I'm 
thinking of the fact that 2x4 boards are not actually 2" by 4".  I 
suppose that a 24" fixture is essentially what I have now, but what 
about bulbs?  In this catalog (_That_Fish_Place_), they have 17", 18", 
and 24" lights.  Are they measured pin ends to pin ends (I have bi-pin 
lights)?  Or is the "true bulb" only what you measure?  If I only 
measure the true bulb, mine are 17".  With the pins added, it is about 
17 3/4".  The other thing that I've considered is that the bulb is 
named for the fixture in which it fits.  In which case I would want to 
buy 24" bulbs for my ~24" fixture...
	Uff-da!  I hope that someone can help me with this.  The leftover 
bulb boxes that I could find don't have the measurements anywhere on 
the box.  I'd hate to buy the wrong thing through mail order.
	So, to light one half (2 feet) of my tank, do I need:
24" strip w/18" bulbs,  <---my guess
24" strip w/17" bulbs, or
24" strip w/24" bulbs?
	Thanks for reading this lengthy and, perhaps, dull post.  Thanks, 
also, for any advice.

Shane E. Wernsing
wern0047 at gold_tc.umn.edu