how to chose the right frame size for touring bike? I am going on a two week bike trip with a friend and I can only afford a second hand bike. My concern is that I will wind up buying a bike that I later discover was the wrong size for me.

I know that in an ideal situation I would try out a bunch of bikes at a shop and pick the one that fits the best, but if I am buying one second hand it's going to be more of a, 'ride it around the block and then make a decision' scenario. 

Given that, are there some tips / things to keep in mind that would help to lower my chances of buying an ill-fitting bike?
How:

The low tech way:


Measuring tape Simply take it with you and measure five bikes that are comfy, and five that are not.
Digital camera Ask someone take a picture of you sitting on the bike, then review the picture to analyse yourself the sitting positions, both those that feel comfortable, and those that feel uncomfortable.


Geometry:


Chainstay. You need long enough chainstay for panniers. My recommendation is 45 cm. Below 42 cm your feet can hit the panniers even with small feet.
Comfort. You can select the ideal frame size based on different criteria: aerodynamics, efficiency, comfort, resulting speed, etc. For touring, the comfort is 10x more important then speed or anything else. This is because you sit 3-6h a day, for many days. On a tour, you get further by pedaling more and less by pedaling faster. Example: you can be 30-50% faster in an uncomfortable, but speedy position, but you can maintain that only for 1-2h. However, in a comfortable position, you can stay even 6-7h in the saddle.


I'd rather select a slightly smaller bike vs. a slightly bigger bike. 

By buying a second hand bike, what you risk is hidden failures vs. the gain of lower cost. 


Steel frames: check for "bents" and corrosion
Alu frames: check for cracks.


Bents usually come from accidents (falling with the bike), and cannot be corrected. You feel it only in the first moments of the riding, after a couple of minutes the feeling can go away. This cannot be repaired. I do have a steel bike that is bent.

Cracks come from big impacts and overload. This can be corrected by welding the alu frame, you need to find an alu-welder. I do have an alu bike with a crack (high quality brand, I didn't take yet the time to find a welder and get it repaired.

For more details, see my response here:
How do you distinguish a touring bike...