Question about painting over a scan

[Delfin93]

Thanks a lot guys! I will try it out this weekend and then tell you what worked out the best for me!

And I also have another question:

I want to print this drawing onto a tshirt and it would be very helpful if I could find out how big the drawing will be (in centimenter) before I print it.
Is there an option in GIMP to show me the actual size if I print it out on DIN4 paper?

Thanks a lot in advance!