If you can take only one fly rod, I'd suggest a 9' for a 9WT line.
An 8WT will cover a lot of bases too, but whether you choose an 8 or 9, make sure you have a good drag system and at least 100yds (I like 200) of #30 backing. If you've never fished salt, it's an entirely new game - fish are much stronger and will amaze you with how far and fast they run......like nothing in freshwater her in PA ('cept maybe a big carp on an open river). Salt game fish really pull!
Frankly, there are so many gamefish and so many different ways in which one can pursue fly fishing in the Keys.....no single rod can cover all the bases. This time of year - mid March - is a bit early on the flats but things should start picking up. You can (and probably will, esp if you use a guide) see bonefish, permit, small tarpon, jacks, small sharks, snappers, and cudas. A 9WT will handle all of them handily as long as you're wise enough not to try and hook big sharks (big is relative, on Keys flats I'd define that as any BTs or lemons over about 3-4' long, although they get much larger than this) or tarpon over 50 or 60lbs.
Out on the reef you'll be targeting bigger critters and a 10-12WT rod would be better......but for flats, I'd recommend a 9WT or an 8WT if you mainly wish to target bonefish.
If you can take two rods, make 'em an 8WT and 10 or 11WT.
Fly fishing in the Keys is complicated (and challenging).