That isn't the question you need to answer. Your potential future singlespeed can be literally whatever gearing you want, so what you need to determine whether you want to be able to shift or not. Riding a geared bike around is a great way to figure what gearing you like, just set it to a good all around gear and leave it. If you like it, note the number of teeth on the cog and chainwheel you're using. People usually start around 70 gear inches and go from there- Sheldon Brown has all the info you need about selecting a ratio.