Nelson powertube assemblies.

These tubes fit nearly all nelson markers. Sizes range from -2 up to 6. A smaller size means a smaller inner diameter air passage.
A smaller size will restrict air flow and choke the amount of air used to fire the paintball resulting in a lower velocity.

The sizes (-2 to 6) range from id .10 to about .19. With the -2 and 0s being around .11-.12. and size 6 is usually around .18 or so.
Size 4 is about .65-.75

It is important to remember that different manufactuers (Taso/AGS, Lapco, Line SI) probably held different standard for sizes so a #4 from Lapco might not be the same as a #4 from AGS or Line SI. This is why I try and list the inner diameters for each tube.

When I'm setting up my loaner nelsons I typically start with a number 4 and use red main and yellow valve with my adjuster screwed all the way out. This will typically get a nice velocity right off the bat (around 250). And from If my velocity is too high with this combo I will go to a mainspring with less tension. If you don't have the option to change main springs then you can change down powertubes.