I have a Fuji S12-S that I have decided to turn fixie. The frame is, i think, a 61 or 62cm frame, and i was wondering what size crankset and cog should i buy? whats the difference between different sized cogs? Same for cranks.

re: cogs etc. :

https://www.bikeforums.net/singlespeed-fixed-gear/468351-gearing-primer.html

Go with a 46x16, its a good all around gear ratio. There is sticky post about gears.