What are the definitions for LPARAM and WPARAM?

Here:

typedef UINT_PTR WPARAM;
typedef LONG_PTR LPARAM;

LPARAM is a typedef for LONG_PTR which is a long (signed 32-bit) on win32 and __int64 (signed 64-bit) on x86_64.

WPARAM is a typedef for UINT_PTR which is an unsigned int (unsigned 32-bit) on win32 and unsigned __int64 (unsigned 64-bit) on x86_64.

MSDN link

These typedefs go back to the 16-bit days. Originally, LPARAM was a long (signed 32-bit) and WPARAM was a WORD (unsigned 16-bit), hence the W and L. Due to the common practice of passing casted pointers as message parameters, WPARAM was expanded to 32 bits on Win32, and both LPARAM and WPARAM were expanded to 64 bits on Win64.

In C#, you should use IntPtr for LPARAM and UIntPtr for WPARAM.

Note that despite the LP prefix, LPARAM is not a far pointer to an ARAM.

LPARAM refers to a LONG_PTR and WPARAM refers to a UINT_PTR

On x86 they will be 4 bytes and on x64 they will be 8 bytes.