I chose to set the framing for the floor on heavy duty tripled 2x10 girders, each 25' long. I run the girders this way, so that the floor joists run perpindicular above them go the shorter 16' distance. In large homes girders are used to support the floor and are run midway between block walls. In almost all scenarios they are composed of tripled 2x10's.
Many of the owner-builder designs for small cabins that I found went with only doubled 2x10's. One even a doubled 2x8. But lumber is really cheap this year, in fact the cheapest it's been in 30 years, due to the recession - this is what they tell me when I order the wood. Whereas my first year up here building, a 16' 2x10 was around $15, this year it's only $9. That's a major price reduction, and will save us a ton of money. And as far as tripling up the boards, you can never go wrong with a little overbuilding, if that's what it is. These beams aren't just functioning as supporting girders, they're really more like 'sills' in old log cabin designs - they are taking the entire weight of the building.
I've asked for the straightest lengths of 2x10 they can get me - little crown, no splits . . . and in general that's what I get. Pretty good boards.
Here Brooke's helping hold one of the boards upright, as I go around clamping it before I fasten: