What is meant by a letterbox logo ? the client asked for the logo to be more like a letterbox than a portrait or a landscape so what does it mean for the logo to be a letterbox?
When talking about letter box they are referring most likely to the shape of the design. Less of a square more of the shape of an envelope. there is no specific ratio but think about the shape of an envelope almost like a 4:1 width:height ratio or higher.
Look at the GD:SE logo above as an example.
at least this is my interpretation.
Just think of the shape of a letterbox on a door