I'm self-building a home, and I'm trying to figure out how to size I-beams and/or roof trusses properly.

From wall to wall, the interior of my home is 30' 1.5". This consists of two spans measuring 16' 3.5" and 13' 5". I'm trying to figure out two things:

1) How long should I tell the truss manufacturer / I-beam distributor to make the trusses/I-beams? Exactly 30' 1.5" or a little smaller, say 30' 1.25"? Can they be shortened in the field? (I'm particularly worried about the trusses)

2) Am I reading span tables correctly? So for example the span tables I got from one truss manufacturer list that a 12" deep truss, spaced 24" o/c and loaded with 40 psf live load / 15 psf dead load can have a span of 17' 7" with deflection L/480. Since I have two spans of 16' 3.5" and 13' 5", that truss would be suitable for me, right?

I'm also trying to decide between I-beams and trusses. Trusses sound more attractive to me because of the ease of installing plumbing and electrical.

talk to your I joist and truss supplier - they have computer programs supplied by the manufacturers, and trained designers who can size the components. the only other way is to hire a structural engineer. If you size them yourself and have a problem, you are on the hook.

You should look at "trimable" floor trusses such as spacejoist if you are overly worried about the length and your house squareness. Some truss companies do their own trimable trusses.