If you are thinking of using something like implement enamel you would be better with the 1.8mm nozzle as it is very thick I will usually use 2mm nozzle to help keep paint wet and flow together, if you spray too dry then you end up with a rough looking orange peel finish.

Once you get set up grab some cardboard and do some practicing, try one nozzle see how it sprays try thinning a little if necessary as enamel does not like being thinned over 10%. You can play with airpressure some too as I find some paints like more or less air to get a smooth finish and play with the speed you move the gun, look from the side as you spray and watch how the paint lays down. For heavy paints like enamels or clear coats I will run the fluid control wide open.