What S-Express says , as long as you have room just move the wheel back in the dropouts to take up chain slack .
I use my single speed for cross and road . It has a 21 on one side and a 16 on the other and I can use both by moving the wheel in the dropouts.
If you find it's still too slack with the wheel as far back as it will go remove a link from the chain and try again . Repeat until happy .
Obviously if you have vertical dropouts ( and thus can't move the rear wheel)you will need to remove links from the chain to get as close as possible to the desired chain tension