Nip down to your nearest appliance store with a tape rule - a notepad and a pencil. In Europe measure in mm and in the USA inches I guess.
Measure the hole you have - you say the space was for a space-saver model - that suggests a size smaller than normal and that suggests more than one size.My cheap Chinese microwave (1995 vintage) is 18" left to right - 14" front to back and 12" bottom to top.
Specifications are not kept here - nothing is kept here...
One inch is 25.4 mm
Good luck...
April 2015