Serotta dealers will size you up on their size cycle, but if your going to go custom then the process is much more extensive and not included in the price of your frame from most dealers, unless your going to spend a grip of cash on components and wheels through the shop it's anywhere from $75 to $200 for a fitting. I would go custom just for the fact that you can have the ride characteristics you desire in a bike more so than the size.
I can ride a 55 or a 56 and make both fit but I'm not a racer so I prefer a neutral steering angle on the headtube but I like to climb out of the saddle so a stiff bb is my choice, and I'm 185lbs so all that is taken into consideration, that's what makes custom worth the extra dough. I currently own 2 legend ti serottas( 1 compact and 1 flat tt), but neither were built custom for me, my favorite bike is my custom steel kirk, hands down.