Re: size of the earth's magnetic poles on the ground, effects on compass

The magnetic pole is A POINT in the Earth's surface where a magnetized 
needle will stay in vertical, with inclination of 90 degrees. Well, you 
can't use a compass in this region because the needle will tend to be in 
the vertical, and you will not read it. Both the poles are the 
same "size", if you consider the dip poles (i.e., the point in the Earth's 
surface where the needle points downward or upward in the vertical). 
The magnitude of the magnetic force can be different (the "strength") in 
both poles. Remember that the Earth's magnetic field is not a dipolar one, 
as a bar magnet, although it can be assumed as an excellent approximations 
(a dipole can explain more than 90% of the Earth's magnetic field).

In practice, to find out the place where the magnetic pole is, we have to 
use some smart strategy; a very good reference to the story of the 
expedition that found the north magnetic pole can be found in an article 
of the NEW SCIENTIST, 4 june 1981, called "Tracking the north magnetic 
pole", by Dr. Paul Serson.

Best regards

Eder C. Molina
Dept. of Geophysics
Institute of Astronomy and Geophysics
University of Sao Paulo - USP
BRAZIL