Concerned about board length

Now I'm in an office job, have gained 20 lbs. and will only ride 20-30 days this year.   I expect that I will not regain the leg strength I had while working the resort.  I've ridden my 159 and slightly shorter boards 155-156.  I find my legs don't tire as quickly on the shorter boards.  I'll be sacrificing some of the speed but when I buy a new board this year and I will likely go with a shorter board.

My mate has tree trunk legs, is a proper full on gym bunny. Hes knackered snowboarding far earlier than me. He has incredible leg strength and can out squat all of us, but on a snowboard hes done way earlier than fat little unhealthy me. I ride more than him. Endurance v burst etc. Never mind "leg strength", the question is do you have your snowboarding muscles? And hows your general fitness level? 

And then it gets more complex...

You see - common knowledge suggests the following:

1. Size down for less swing weight.

2. Size up for more float.

3. Stick around your regular size for pretty much everything.

Obviously certain things complicate matters (the nug for stability/swing weight and reverse camber for float), but its the general idea.

Now consider this; you suggest that you should size down for less fatigue, but what kind of riding are you doing? Are you riding in any powder? Hows that thigh burn working out for you ;)?

How is your stability in chop? Are you getting bounced around a bit more? Are you washing out? Hows the shorter effective edge and/or lack of contacts/too many contacts working out? 

Got dampening? No? Hows it feel being bounced about all over the resort and feeling every impact with the terrain?

Suddenly me on my longer damp board is just plowing through the chop and crud, not fighting my edge, definitely not washing, and certainly not being bounced around. Straight lines, beautiful pin like carves, and all with zero effort because its what the board was built for. So now what you have is that shorter board working against you. The longer board on the other hand is utterly effortless. You dont need to make sharper turns like you will on a shorter board, you can complete trust its edges and just let it run... whereas on a shorter board you might feel its losing a spot of stability so continually have to reign it in. Suddenly the longer board is causing you to burn much less energy than the shorter one. Youre simply having to do less to keep it under control. Its designed to just rip through that terrain all day whilst the shorter board is forcing you to make shorter arcs, more stops, more adjustments, more effort all round. And this doesnt even broach the subject of powder... where its not even micro differences that build to significant ones over time. Its huge. Smaller board area = more fatigue and possibly much longer hikes out of the snow once you have to unclip and swim back to the groomers. Thats one for really killing your energy ;)  

Id therefore argue that your general fitness simply impacts your ability to ride all of it. The fitter you are the less swing weight is going to bother you on a longer board, and the less chop, crud and shitty lines are going to knacker you out on a shorter board. Basically, good fitness is always an advantage to just being able to go longer, this isnt really up for debate, but as for the question of whether it should impact your board choice and size in general, it opens up the question of "what do you like to ride?" which then brings us all the way back to square one. Once they answer that question, and once you know their size, you can generally give them a fair idea of their size. 

And this is where you end up back where you started at the "common knowldege" part above. :) I think this is pretty much why fitness isnt really an issue in sizing. You pick the size for the job. If you want to spin around all day and since swing weight is going to fatigue you, go shorter.  If on the other hand you want to rip through choppy messy lines and hit up the black diamonds, then a board with a nice long edge, decent contacts and some solid dampening is going to make it absolutely effortless out there. If instead you just want a bit of everything, then you know... pick up a bit of everything style board with a decent pliable flex, good stability, and some decent float. You know, basically all the stuff back at square one :)