I'm planning a cedar pergola over my deck. Posts will be 6"X6" and will be notched to accept a "sandwich" beam of two 2"X?"s. Across the beams, the rafters will be 2"X6"s on ~8" centers, which should be sufficient given their 4~6' spans. If I need a beam to span 13~14', will a pair of 2"X8"s sandwiched around the posts be sufficient? I assume that 2"X6"s would be too small. I live in Colorado so wind and snow loadings are in effect. The pergola may someday support vines. I've found some online span charts, but they don't really apply to pergolas, which carry so much less load than floor/ceiling framing. Any help on the beam size is appreciated.

Pergola Beam & Rafter Sizes & Spans