Hiya BCwoodtick and welcome to the site!! Always nice ta have another PNW'er aboard
I spent a bit of time doing timber framing in the late 80's and recall seeing several tables for shear/loading stress etc. for different species and spans but the sources have long escaped this seive I call a brain. I believe they were Canadian gov't sources but I am unsure what the heck they were (use it or lose it lol) so, just because, I hunted you down a source for free PDF files that deal with many aspects of timber framing and I believe some of the available tables will have the info you require. Hope this helps!
American Wood Council links and PDFs here>
http://www.awc.org/wood-design/archives/2006_09_01_wood-design_archive.html
Specificly in this section>
http://www.awc.org/Standards/wsdd.html
If I find more on this I'll add it to this thread.
Speaking of which (it is a huge subject btw, mine eyes is googled out) here is another site with multiple international links including to CSA and ASA sites, the rest of the reading is up to you
http://www.alexschreyer.net/timber-engineering/codes-standards/
Your talking a pretty large free span if it is a central supporting beam for your roof trusses, you don't mention the width of said structure either, so you'll have do do some calculating as to the weight it will be supporting, snow loads, etc. etc., a lot of variables involved icluding supporting its own weight, so it makes me think, um, big, like 8x 16 or better if pine but I am only guessing. Let us know what ya come up with and post some pics, we loves pics!
Serge