How to select cog for emergency single speed conversion with a broken derailleur
The recommendation of Calvin Jones of Park Tool for a trail/road-side conversion to a single speed is:


If you have multiple chain rings, put the chain on the 2nd-to-largest chain ring.  I.e. if you have two chain rings, this will be your smallest.
For the cassette, put the chain in a straight line back from the chosen chain ring.  If in doubt, use the smaller of the two choices.
Join the shortened chain outside of the rear derailleur, and as tight as is reasonable. 


This setup will give you a versatile gearing that you can get going without blowing out your knees, and neither the chain ring nor the cassette will be likely jump down to a smaller gear.