modifing a diffuser

[kgm2535]

I need to connect smaller diameter ducts to a diffuser

the smallest 24X24 diffuser is 6" neck in the acad library

can that be changed to 4" neck

can diffuser size be changed to 12X12 instead of 24X24

attached is a 24X24 with 6" neck

thanks

[s.borello]

Can you use 6" to 4" duct transition?