Adjusting gear ratio
If you poke around on the web enough, you'll find a "magical" gear ratio calculator somewhere. You input the length from BB to dropout and it spits out the 
front/back combos that will work w/o a tensioner. 

Try this one. 

http://eehouse.org/fixin/fixmeup.php

It would be much simpler to change the front chainring, if you can find a magical
combo that matches what you want for a gear. There are also "half links" available that allow you to tweak the chain by less than a full 1 inch link pair. 

FWIW, a single speed bike means your almost always in the "wrong" gear. Any change you make will only change when you're in the wrong gear, not the amount of time you spend in the wrong gear. If you're not happy being in the wrong gear, just buy a derailler and a shifter.