I use a boomless on my 4 wheeler mounted sprayer. Your sizing will depend on your pump capacity. I have a 2.1 GPM pump on my 16 gallon sprayer. I sized my nozzle to be less than the 2.1 gpm capacity of the pump to guarantee developing the correct pressure to spray full width. Mine will spray a 10 ft wide pattern and dumps my 16 gallon tank pretty rapidly. I was able to find my nozzle at a farm store. I also agree that high wind may cause more drift with the boomless nozzles. Rate of application is controlled by ground speed.
Listed below is one of many websites available on the internet
TeeJet - Boomless Spray Nozzles