This link may help...
http://www.partselect.com/repair.aspx?appliance=microwave&part=replace-fuse
Most fuses have the rating either printed on the body or stamped onto one of the metal end caps.
A guy on here (microwave engineer) suggested the door interlock switch is a common failure in microwaves - he then suggested that this causes the fuse to blow but the more I think about an open circuit switch causing a fuse to blow the more I doubt his wisdom... might be true.
http://www.howtomendit.com/answers.php?id=231713
There are VERY high voltages in microwaves and dangerous radiation so take great care.
Fuses are rated by Length - Voltage & Current - there are regular fuses, fast blow and slow blow fuses and several other types too - see...
http://en.wikipedia.org/wiki/Fuse_(electrical)
Fuse supplies here...
http://www.cricklewoodelectronics.com/Cricklewood/home.php?cat=202
Peccavi
April 2009