If you have a working geared bike at the moment, the easiest way to find out what ratio you might like in an SS is simply to ride the bike you have and determine which gear you like the most. Then count the teeth.
The easiest way to make it an SS is to STOP SHIFTING.
Really.
You save maybe 1lbs of weight by ripping off shifters, derailers and all that. You WON’T notice that weight while riding.
Also, unless your hybrid has horizontal dropouts, odds are you’ll need a chain tensioner anyhow. The net effect of discarding a derailer in favor of a tensioner is really, really small.