As my question indicates I am a beginner. If true field of view = eyepiece field stop diameter/telescope focal length times 57.3, does this mean that that a 1.25" eyepiece cannot have a tfov greater than approximately 1.25 degrees and a 2' eyepiece a tfov greater than approximately 2.10 degrees?
The maximum field stop possible in a 1.25" or 2" design depends on where the eyepiece designer draws the line for vignetting (darkening at the edge of the field).
Where most 1.25" eyepieces max out at 27.0-27.2mm field stops, there are some that have been as large as 29mm.
Likewise, most 2" eyepieces max out at 46mm, but a few have gone to 46.5 or 47mm.
Note that when the field stops are larger than the inside diameter of the barrels, however, they must be placed very high in the eyepieces, above the barrels, and this necessitates a large
amount of in-travel at the focuser to achieve focus.
But you have put your finger on the WHY of 2" eyepieces--it is to get a wider true field of view than can be achieved with 1.25".
In general, if the focal length of the eyepiece exceeds "X" in 1.25", it must be a 2" eyepiece:
X = 40mm in 40° apparent field
X = 32mm in 50° "
X = 24mm in 68° "
X = 16mm in 82° "
X = 13mm in 100° "
Anything under the focal lengths listed can provide the same apparent fields in 1.25". There is no reason to be a 2" eyepiece.