How to calculate the exact size of a piece of code?

Hi guys,
I'm currently trying to inject some code in a target.
Say that code is a function named void Func()
(I don't want to use assembler, I'm going to coding all
in C/C++.) I use the WriteProcess API to write the code in the target.
The code is written in a buffer allocated using the
VirtualAllocEx API.
I need to calculate the exact number of bytes occupies Func()
so I can pass this information to the writeprocessmemory.
How can i do that?
cheers
z.