Rafter size...2x6, 2x8?

[jp maine]

Building a gable roof with ridge board on a 12x16 room, not exactly sure of pitch of roof yet. Span is 12' to the exterior sheathing, walls will be 2x6. I intend on placing rafters at 16" o.c. Will use 1/2 inch or 7/16 inch plywood and metal roofing over rafters in Maine (snow load). Need to decide on lumber dimensions. Any recommendations on rafters and ridge beam lumber size?

Thanks in advance for the advice.

[Moyneur]

Check out this site
http://www.strober.com/template4.asp?pcode=26&pp=5
Might help, if any a good reference.
http://www.strober.com/lumber_span_table.gif