Your sizing is fine. 5 foot 6, you should be on a 173cm.
Where I was coming from: people may be, on, say a 170cm frontside ski (Head Titan or Volkl AC50 for example), and therefore think the 173cm is the right length if they buy a Brahma, especially if they are getting into a more rockered ski for the first time, or are coming out of the Supershape school of thought, where everything is 170cm or shorter. Unfortunately, that 173cm will ski short, like being on a 163cm AC50, due to tall tip and tail, plus the longer early rise before the steeper tip and tail rise. On a stiffer frontside ski, most people go head height to -10cm head height: rarely above, as the skis tend to be stiffer and more fun in shorter lengths. On a ski with more rocker/early rise, and a generally softer profile, the skis should be purchased longer: head height if you want a really nimble feel, +5cm or more if you want something a bit more stable. I skied the Brahma in 187cm and it was easy to handle: this ski skis a little shorter than most skis in this range, although I would buy a 180cm (which is +5cm above head height).
As a general rule, you can assume the Brahma will ski 5-8cm shorter than a comparable nearly fully cambered all-mountain more frontside ski (if we are using Blizzard, it would be the Mag 8.5). It will be comparably 9-13cm shorter than a stiffish frontside carver (Supersonic IQ or S800). Regarding the Bonafide: ski the same length in the Brahma as you do in the Bonafide. In the Cochise, there is longer early rise tip and tail, and more of a tallish tip, so you go up again to the next 1/2 size, for the same feel and equivalent length. This is all based on trying to compare apples to apples. If you want an Eastern tree ski, consider sizing down if it makes you more comfortable; if you are ripping wide open terrain at Snowbird, go up.
Here are the lengths I ski, at 5 foot 9, for reference. The preferred length feels "like it should" given what the ski was designed to do.
Supersonic IQ: 167cm (have skied the 174cm, more GS and less energy than I like)
Magnum 8.5ti: 174cm (181cm is too GS-like and boring)
Brahma: 180cm (best all-around length)
Bonafide: 180cm (I grab the 187cm in pow and if I am skiing fast)
Kabookie: 180cm (could go 187cm, but this is my tree ski)
Cochise/Scout: 185cm gives me the float I need while still being nimble