Does anyone have any suggestions on how to handle too much coding for macro?

Under the macro coding, the structure is shown below

Sub main()

A set of coding for A class
....
A set of coding for B class
....
A set of coding for C class
....

End Sub

When there is only coding for A class, there is no problem on running macro,
but when I insert the coding for B class, it pops up something like too much
coding for macro. Does anyone have any suggestions on how to split coding
into different module? so I still can run macro by referring to different
module.
Does anyone have any suggestions?
Thanks in advance for any suggestions
Eric