I am curious about the belfast too. I could only find the one, expected definition.
A metre is generally too wide for a single door so you will likely want and need to split the gap in two. The center stile should be 2" if the sides are 1 1/2 ". A 3/8" over lap leaves a 1 1/4 " reveal on all sides then. In metric that's 51mm, 38mm, 10mm, and 32mm rounded off. You may want to make the top and bottom members wider as this will strengthen the box some.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.