I've gone about the length of my motorcycle pants just like my other pants, if they look right standing up then all is good. I've never paid attention to the length of my pants while on my GSA until today. When it was on the center stand, I sat on the bike, put my boots on the pegs and looked down and saw the end of the pants were about 6 inches above the shoe part of my boot. After thinking about this, it makes more sense to me for the pants to be longer while standing and closer to the shoe part of my boots when riding. This would give more protection while riding. Fellow inmates, what are your thoughts on this?
The short answer is way longer than you would think. Your pants will ride up considerably once you are comfortably positioned on the bike for riding. If you don't want wind and rain crawling up your leg while riding you will need to get pants with a length that is several inches longer than normal for just standing. You get to pick - funny / uncomfortably long while standing or too short for riding. Investigate this before shelling out big bucks for riding pants.
Actually, I have a quite short pair of leathers. My main point of interest was if the knee armour was in the right place when sitting on the bike. Length was not really an issue because I wear my boots over the pants, and with a pair of Oxtar infinities, that's a lot of boot on your lower leg. And with a nice gore-tex liner they ensure that any water that does come in, stays in! So although I can't comment on length, I do want to contribute in that you make sure that the knee armour is in the right place when sitting. Should require only minimal shifting to get them to seat properly on your knees when sitting down.
+1 When I buy strides I make sure I've a bike (or a chair) to straddle - just to make sure the leg length is good for me - good for me is ankle length when sat on the bike. Another poster made a good point about knee armour - I've yet to have any that sit in the right place or give me the confidence that they'll stay put in the event of an off, so I use strap on knee armour from Forcefield - they've allowed a lot more flexibility what in what trousers I wear and I'd recommend them whole-heartedly.
Cuffs about at your ankle bone when seated on the bike. Suggest you look at pants that the maker can alter to give you this, as well as correct armor placement. Might not be "cheap", but they'll be right.
My Aerostich AD1 pants had the knee armor placement adjustable via velcro. I was able to get the armor adjusted just right after some trial and error fitting.
One thing nobody has mentioned yet is getting pants that don't have too much drop. I.e. you want pants that hug your crotch. If the crotch of the pants is 4" below your actual crotch when standing, then when you sit down on the bike and the seat pushes the pants up against you, it's going to pull the legs up 4". My Motoport mesh Kevlar pants were custom made using a good-fitting pair of jeans as a template (which is how they do it for everyone). They fit well in the seat/crotch area. And, when you send them the jeans to use as a template, they have you put the jeans on and sit on the bike and actually use a grease pencil to mark where your knees are, when in the riding position. As a result, the knee armor is placed exactly right when I sit on the bike and the pant leg comes all the way down to the top of my boot (when I'm on the bike). And when I'm off the bike, they don't have that "too long" look. Like I said, getting a good, not-baggy-at-all fit in the crotch is key to keeping the pants from pulling up when you sit on your bike.
