Hello!
After reading your post and recalling my own experiences in determining what to buy, I thought I'd weigh in with my thoughts and share my opinion.
Since you are 172.5 cm in height and 54kg in weight, a 168S would seemingly be the right fit. But like Tali, I do think that perhaps a 161S may be more appropriate.
Weight: 54kg
-168S is rated for riders below 66kg. BUT the 12kg differential of 54kg body weight from the 66kg reference point is pretty substantial. My concern would be that the 168S could possibly be just a tad bit stiff.
-161S is rated for rider weights 46-55kg. your weight falls correctly within this range.
Based on the parameter of weight, I'd say 161S
Height:172.5cm
-168cm would work well.
-161cm should still be considered since rounding your height off at say 172cm, you are right at the borderline between the 161 and 168 board lengths.
-as a reference point, Patrice is 177cm height and rides a 168(difference of 9cm) Jacques is 186cm height and rides a 175(difference of 11cm). You are 172.5 height, a 168 is only 4.5 cm difference, a 161 puts you right at 11.5cm difference)
Based on the parameter of height, I'd still go for the 161, although the additional length 168 would arguably be a bit more stable at speed.
Boot Shell Size:UNKNOWN
This is the BIG question. This factor may well close the case for you in determining which board to buy.
BOOT SIZING/FITTING:
Go buy a metric ruler to precisely measure your foot length to determine Mondo Point size. DON'T attempt to convert your regular street shoe size to a Mondo conversion chart. I've talked to many experienced ski bootfitters and they all tell me that converting street shoe sizes via these Mondo charts very often leads to incorrect sizing. After determining your Mondo point by actual foot measurement, go visit ski shops and try on ski boots of that Mondo size, they should feel very similar to what an alpine snowboard boot will feel. Yes, wear appropriate ski/snowboard specific socks, regular socks for summer sports isn't gonna cut it. Skking socks are a safe bet, since an alpine snowboard boot fits very much like a ski boot.
BOOT SHELL LENGTH:
Once you have a pair of properly fitted boots, you can then measure the Actual Boot Shell Length, as illustrated on the SWOARD site. The key here is to get as low boot angles as possible without toe/heel overhang.
-if your boot shells measure 291mm or shorter, then the 161S would be the clearcut winner, considering your height and weight.
-if your boots are greater than 300mm, then the 168S would be back in the picture.
All said and done, I would advise that you make your boots your top priority at this point in time. Once you have correctly fitted(and probably spring-modified too) boots, you can then measure shell length and factor that in to arrive at your final board choice.
here is a useful link regarding bootfitting from BomberOnline
http://bomberonline.3dcartstores.com/Bo ... _c_22.html
Then click on HOW TO FIT MY BOOTS
Good luck

Riding softboots since '96. Saw EXTREME OPUS 4 "LIFTED" in 2010... Back to Square 1