You start by taking note of any snow, wind and seismic loads as required by your zoning and building codes. These things are determined by code, not your anecdotal evidence. Determine if the member is a ridge beam (supporting half the rafter loads) or a ridge board which is just a place to line up the rafters. Use the actual dead loads for all the members, roofing and sheathing, insulation and ceiling materials and lighting and any other anticipated live loads for storage above the ceiling. Do the calcs and you're set. By the way, gluelams are NOT the thing to use for a ridge beam. Microlams or LVL's (Laminated Veneer Lumber) as they are called are much preferred. If you have a ridge beam, then you need to transfer those loads down to the ground ate each end of the beam. If an opening occurs under one end, like for a garage door, you will have a large point load to deal with for your header.
Oh,
You have already checked the zoning requirements based on your zoning, including setbacks, height and area requirements etc., looked for utility and other easements on your mortgage and any other surveys, checked into homeowner association rules (if any), and looked for deed restrictions, right?
And that 3/12 pitch.........I like to get 12/12 to shed rain, much less snow. Gives storage above the ceiling joists too! Also think about design that will complement the main house and surrounding structures and the environment it is in. Think about window and door placement to give you pleasant outside views as you work and efficient materials handling from purchase, storage, manufacturing and back out of the building. Think about maxing out the height for more usable spaces. Think about making the space expandable for future uses. Think about adjacent paving for outdoor workspace. Think about lean to roofs for outdoor storage. Think about skylights for light and ventilation. Think about as big a door opening as the structure will allow to accommodate big projects. Etc. Etc.
When you get all the requirements from zoning, code etc., if you are not required to get any engineers stamp for the structure, you will proceed like this to size the ridge beam.
Take the total live and dead loads of the areas contributing to the beam. Figure the load/LF on the beam. Find a table that has the deflection you want to allow, say 1/240 or 1/360, and look for the LVL or combination of LVL's that will meet or exceed that load. Note the Fb and species and manufacturer and get that beam. Note that with a ridge beam the ceiling joists and how they are secured to the wall top plates, and the rafters and how they are secured to the walls, are as important as the beam. I like to use metal fasteners to secure each member to the next. That way the joists don't allow the walls to spread, allowing the joists to spread and the ridge beam to become sway backed. I use joist hangers at the rafter to ridge beam, Simpson connectors at the rafter tails, and also at the ceiling joist to wall connection. You may have additional seismic requirements for connecting the wall framing to the foundation. Also be careful when you have a large opening for a door with small areas of wall left on either side of it. You may need to create shear walls to prevent racking and collapse of this wall from wind and seismic forces.
