is it best to ride a bigger chainring / bigger sprocket or vice versa (e.g. 48x19 vs 42x16)
do you ride 1/8 or 3/32 chain
are sprockets easy to come by in your neck of the woods - some people look at me gone out when I ask about fixed sprockets here....
If you're doing a road conversion, use whatever inner ring you've already got and pick the sprocket accordingly. Sprockets are most common smaller than 18 though, and if you can find them the bigger ones usually cost more. Also big-big means more chain and heavier.
I use 3/32, again more common.
I mail order the gear from places that pride themselves in being fixed friendly.