The very frequently used Eurocard format describes a standardized size of printed circuit boards with dimensions of 100mm x 160mm and the thickness 1.6mm. Also common are boards in Double Eurocard format with a size of 233.35 mm x 160mm and Half Eurocard with 100mm x 80mm. This is regulated in accordance with DIN 41494, IEC 60297 and DIN IEC 249 as well as IEEE 1101.10.
The aim of standardization of PCB sizes in Europe in the 70s was to define fixed metric units for the previously inch-based measurements. Additionally, manufacturing-machines and -panels could be standardized and thereby made more economical.
The Eurocard - and Double Eurocard format is a often used slot size for racks and other casings so that for these formats the largest range of products is available, which in turn leads to their frequent use:
Eurocard (3U*) : 100mm × 160mm, contacted on the narrow side
Double Eurocard (6U *): 233mm × 160mm, contacted on the wide side .
*U = Rack Unit, more details see "Eurocard" on Wikipedia