I'm still incredibly green to Ver.3 so you'll these noob posts pop up once in awhile from me

Still have a hard time taking in info from outside of ASC and always do so with a grain of salt.
I'm currently building up a CYMA gearbox - i've paid 40$ for it and its a beater but I figure its a great learning platform.
I'm looking at replacing the cylinder head right off the bat along with a few other parts - now when im browsing about i've come across two different kinds, the long cylinder head and the short.
Is there a reason for these two kinds? The original stock cylinder head was a long type, however the actual stock nozzle was surprisingly short and very similar to an M4 type.
Nozzles also come in two different sizes..Do I have to abide by this??
What if I mix and match?