Frame size

  Gazelle Champion Mondials are measured center-to-top or c-t as it is commonly abbreviated. Center-to-top means that the frame is measured from the center of the bottom bracket, along the seat tube, to the very end of the seat tube. The pictures to the left show a size 58.
It stands to reason that the measurements are taken in centimeters, not inches, as cycling is a metric sport.


To avoid any confusion, later Champion Mondials have the frame size stamped on the rear brake bridge:

Late seventies, early eighties 'butterfly' frames carry the frame size at the rear of the brake bridge, left of the brake while the rear axle spacing is shown at the right side of the brake. Here a size 58 with a rear axle spacing of 120 mm (5 vit.)
 
Later Champion Mondial road frames had this Cinelli-type rear brake bridge. The frame size is generally stamped at the underside of the bridge, only visible from below. Here a size 59.


Size
(center-to-top)
  Top tube
(center-to-center)
Contrary to most Italian frames, Gazelle's are not built 'square' (i.e. seat tube and top tube having the same c/c dimension). Instead, up to four sizes use the same top tube length.

The 1991 brochure has more information on frame dimensions. As far as I know sizing changed little over the entire period that the Champion Mondials were produced.

49-50 cm 525 mm
51-52 cm 535 mm
53-56 cm 545 mm
57-59 cm 560 mm
60-62 cm 575 mm
63-66 cm   585 mm