I "Borrowed" this off another forum.
Tip tops are sized in 64ths of an inch, so a 4.5 tip top is 4.5/64 or .070". Here are the standard sizes and their equivalents. Please note that this does not refer to the diameter of the loop on the tip top, just the part that fits over the rod blank.
For reference 1mm = .0394" or 1" = 25.4mm