It's not located on/at Mars, it's located at the Lagrange point - which means much like your hand can cover entire mountains at a distance, a smaller magnetic field could shield an entire planet when positioned between mars and the sun.
Based on the drawing that they made the size is at least 1-2 diameters of mars (but there is no field there?). Since the diameter of Mars is ~4000mi that would >10,000km at 5mT.
So with the mentioned 1-2 Tesla starting field, we end up and a cube law reducing the strength 200x at 6 times the distance, and a 1.5km moment. That would require at least 2-3km^2 of 1T magnets. Ignoring the support structure that would have a volume of ~10^10 cm^3 and weigh 100ktons, but only cost $100B on Earth at todays Samarium Cobalt pricing, if the market wasn't highly distorted.
