I think they are pretty similar to German Shepherds, but Malinois are leaner and more agile, Shepherds are stockier and tougher.
The German Shepherd Rescue of Orange County (gsroc.org) will have 1-2 Belgian Malinois at any one time but none right now.
Typically they have a higher prey drive than GSDs. Our rescue will allow someone to adopt a Belgian Malinois only if they have had experience with GSDs, never a first time adopter.