Sizing of timbers is based on lots of information.
You'll need to collect this information and have it ready when you contact a timber frame engineer.
An experienced timber frame engineer can do this, to find one, use the resource guide, select the icon on the guild home page, and follow the links to the search page, do a search and get a list of engineers for your area. They should be familiar with the requirements you'll have to meet.
Good luck,
Jim Rogers