Do I have to declare the stack pointer?
Not necessarily, it defaults to 07H after reset.
How many bytes of memory do I need for the stack (for CALL function) anyway?

Well, it depends upon the level of nesting. For one call you need only 2 bytes, and when you return from that call, you are again at 07H.

One of my codes (over 10K binary) was written in assembly, and i just used from 07H-1FH as stack. I had no problem.

IA