In searching for info on beam sizing I ran across these 2 pieces of info.
First, a program called BeamChek. I'd seen this before but never looked at it much as the price tag of $150 scared me away. There is a free trial available, tho' it is crippled as to beam span. I downloaded it tho' and found I was a bit crippled as far as being knowledagble enough to use it. If you want a free look it's at (You have to some some info for registration, an email address is one thing. )http://www.builderswebsource.com/software/beamchek/software.beamchek.htm
In the email they sent out saying thanks is a link to a free download for a PDF document entitled "Principles of Structural Design". It's a good explanation, with diagrams, as to how roof, floor, wall loads are transfered to the foundation, and more. Not a course on engineering, but it might make understanding your buildings loads easier. As I said it's a free download but the doc is copyrighted with a disclaimer against unauthorized distribution, etc. so as much as I'd like to post the direct link, if you want to have a look you'll need to go to the above link and follow the steps. It's no big deal.
Here is a free online course that may also help understand some of this. Wish I had time to go through all of it.
https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=st&chap_sec=01.0
At the top above the window are more links in each section of the course. Check out the video and audio.