there are different ways of running it...
you could run a single shot with a momentary switch and hold it down whenever you want.
there are pedal activated switches that spray when the pedal is pushed down at WOT
there is another an nx progressive controller. thats what i have. you can run 2 stages that way and have then ramped on whenever you want,
jet size depends on your truck, make some passes and keep changing jets till you cant go any faster (too big will slow the truck down)
if you want a good starter kit that you can build off of then call up NX and look into their stacker kit, its a single stage with a pedal activated switch. its a nice kit that will give you some extra hp.
for anyone running bigger injectors then you can obviously go up in jet sizes because you will have more fuel to supply it. 1 or 2 stages with proper jetting and you can fine tune with the progressive controller.
the way the progressive controller works is boost reference and wot . the pedal activated is wot and the momentary switch is when you want. but at the truck while you are staging once the light turns green hit the go pedal and the momentary switch and just hold it. its a cheap and easy way to hook up nitrous. I run a 15lb bottle with a single stage and dual stage setup.