They are sized based off of goals and use of the truck.
Stage 1's ($1,100)- Usually for the guy that wants a mild bump in power without having to do many supporting mods. Budget minded for the guy that absolutely positively will not want a bump up in power town the road. Usually this guy already has weak injectors and wants to replace them with a better product. Just under 400hp
Stage 2's- Usually for the misinformed........... Yeah that probably just opened a can of worms, but there it is. Yes i said it. Deal with it.
Stage 3 hybrids ($1,850)- There are various sizes at this level and they are all the same price. You have capacity in cc's and nozzle size in %. Capacity means nothing as long as it is enough to keep up with nozzle size. Since they're all the same price and capacity doesn't really mean anything unless you're completely emptying the injector, I say get 250's. No matter what nozzle you get in this stag, get 250 cc injectors. Nozzle size is what is important. Here you have a few choices as well. The most popular 3 choices are 80, 100, and 200%. It was thought for a while (and still is for some) that a 238/80 was the perfect 450-500hp injector. Clean and smooth. With the capacity thing I just told you about above, that bumps it to 250/80. However, recent advancements in 7.3l performance has led a lot of us to believe that a bigger nozzle is actually better. Its safer because of the lack of needed soi timing advancement, and because tuning can tame the injectors down to near stock levels, you have room to grow. The 250/100 is increasingly popular because the 100% nozzle is about the perfect 500hp nozzle. Some people feel the 100% is better than the 200% in that it is less smokey. If that is the case, you need to find a different tuner. Period. A 200% nozzle isn't quite as tamable as a 100% nozzle. Detuned they still produce around 300hp. However, you can make up to around 600hp with them. All of these stage 3 hybrids will still allow you to tow max gvwr. The only limitations are the supporting mods. Your tuner will tune appropriately for your setup to get maximum gains while still being (fir the circumstances) safe.
In all these cases your budget and ultimate goals are the final determining factor.
Popular hp/injector choices:
375hp- stage 1's
400-500hp- 250/100's
500-590hp- 250/200's