Ever seen sagging roofs? Think swayback. Eventually the material sags under its own weight for lack of support, cracking shtrk, pressing down on the window and prevent it from working and likely crack it from pressure. If drastic enough will also pull material from the side possibly wracking doors and cracking wall and ceiling drywall radiating out from the window.
In all the "OLD" P&Bs and balloon frames I've worked on, built from 1787 to 1940s, the largest top plate was a fir 4X4. Today this won't qualify for a Case 1 structure let alone Case 2.
Case 1 = Ranch or Cape with roof above no 2nd fl..
Case 2 = Any structure with 1 floor above supporting roof.
The minimum for a 5' foot span in a case 1 structure is 2/2X6 with spacer, (nominal hdr. dim. of 5-1/2"X3-1/2" + top plate and tie in plate = 8-1/2").
In general regarding spans on any structure architects err on the side of caution and always bump up to the next larger hdr. whether it be steel, engeneered or common framing lumber. Today a 1st fl. 8' pic wind on a case 2 would require a 2/2X10" or 2/2X12" with 1/2" steel flitch plate. Smaller hdrs would require an engineer stamp for appropriate steel I beams.
All 1st fl. hdrs would be the same, (possibly excluding the front and side doors unless sliders, French or w/side lights). This is done for strength and sidewall continuity, it looks nicer when siding follows all winds and doors at the same hgt.
Many states/towns have "Home Owners Exemption Clauses" where the owners are responsible for following the code and calling for rough, intermediate and finish inspections, (DIY). Cutting out contrsactors and labor does save money but then you have the stress of not knowing the building code and not having all the skills, tools and subs, (plumbers, elects, insul and dry wall) required to complete the project.
New frames are simpler to create, you start at the bottom and work your way up. Remodeling requires structural knowledge beyond a common need, e.g., load bearing walls and how and when to support during reconstruction. Too often one reads about a death do to a floor collapsing on a home owner doing their own work.