The size classes only apply to modern toys, generally Armada and forward. They are used to determine price. some toys may only be available in one size class or many size classes.
for the current line there is Titan Master, Legends, Deluxe, Voyager, Leader and Titan in order of actual size.
If you are planning on collecting actual toys or re-releases of toys from the 80s then you'll be looking at the current Platinum Edition line, and some of the older re-issue lines which again have no size classes. This re-issue lines take toys from the 80's and re-issue them brand new. some will have similar packaging to the 80's boxes other times they will new packaging in line with the line they are being released in.
80's re-issue lines include:
Hasbro's Commemorative Series
Takara's Transformers Collection a.k.a Book Series
Takara's Encore Series,
and most recently Hasbro's or Hasbro Asia's Platinum Edition series.
Prices will vary depending on the figure you are looking for and the specific release or series it comes from, and some are harder to come by than others.
The current Platinum line has:
Autobot Intel Ops, Blaster and Perceptor
Trypticon
Insecticon 3 pack
Predaking
Triple Changers (Blitzwing and Astrotrain)
There have been other G1 / 80's releases not part of a series but as on offs for specific toys. Optimus Prime has been re-issued several times for different lines.
Depending on which toys you are after and your budget will likely determine the series you'll want choose.
You can look in places like BBTS or Tfsource or things like that for the currently available 80s figures.