Size of Pointer

I appreciate this is probabky a really stupid question, but it has me
totally lost.
I am passing a stream via a memory mapped file. The app that needs to
receive the stream needs to know the size of the stream to read.

I am reading as follows:

    Ptr := MapViewOfFile(hdl, FILE_MAP_READ or FILE_MAP_WRITE, 0, 0, 0);

    Ptr is declared as a PChar.

    Not sure if I should be declaring it as such. How do I work out the size
of the stream I am trying to load?

Ptr.size. Too easy

Thanks