Generally, the original Patent will denote the acreage originally conveyed. As you noted, every conveyancing instrument put of record for the subject lands will need to be examined to determine both the surface estate conveyed and the mineral estate conveyed as the ownership evolved over time.
Mineral acreage conveys with the surface unless a) it has been previously, or is currently being, reserved, or b) a mineral deed has been executed which severs a portion of, or the entirety of, the mineral estate from the surface estate. Mineral reservations can be 'in perpetuity', for a specified period, or for other reasons, and royalty interests can also enter into the equation. Those all being reasons that the entire chain of title needs to be examined to determine ownership.
To Jackie and Joyce, accurate determination of the size of a mineral estate requires a lot of investigation. The nuances can put this accurate determination into the realm of retaining a professional who has the experience to accurately interpret the chain of title and arrive at an accurate assessment of who owns what, how much they own, and any clouds that may be found on said title. I'm not saying that it's impossible for a layperson, just more complex than most people want to undertake.
Even Landmen assigned to perform a title package have their assessment reviewed by a title attorney to identify 'curative issues' in that chain of title which require affidavits, probates, etc. before they issue a 'title opinion' the drilling company relies upon to insure they have all relevant interests leased before drilling operations are commenced.
As Darla mentioned, Title Plants generally only tabulate the surface estate. They do have a list of all (most) instruments that have been filed. Some will let you see that list, some won't. If you can acquire or make a copy of that list you can start at the beginning and run the title to current day. Or, if you can locate an oil & gas lease that was put of record at the courthouse, that will give you an acreage for what was leased...usually everything one owns in a section.
Hope this helps a bit.