TBitmap sizing taking into account screen size

Hello,

I'll try to explain my problem in english which is not my mother tongue and
I apologize for the language errors that will contain this mail.

So, I'm currently building a Windows Paint like application using BCB 3.0
and for this I use TBitmap.

I'd like my Bitmap to look like a A4 sheet on the screen (210mm ,297 mm)
whatever the screen resolution is (or more exactly taking into account the
current screen resolution 800*600, 1024*768, ...).

What must be the values of TBitmap width and height to achieve this?
I've heard about TWIPS which are Micro.... things but that doesn'y exist in
BCB.

Thank you