Tributary areas of components and claddings are typically taken as the distance from the span supports times 1/3 of that distance. On a fully adhered membrane, the "supports" are continuous, resulting in a ultr-conservative tributary area of 1 square foot. However, for a membrane to fail in wind uplift, the entire area would have to uplift with it. I would say a tributary area of 10' by 10' would seem appropriate. Lastly, since it is not a structural member, it really falls on the architect (I'm sure he'll have fun trying to figure it out), or on manufacturer's testing data.
The tributary area for a membrane is the area affected or held by a fastener. For example, most codes require one mechanical fastener for every 2 square feet of membrane/insulation area in the field of the roof. This can be used as an equivalent for fully adhered membranes as well.
