Well, when using a refractor you can use any eyepiece focal length that you want to give the widest field and/or the lowest magnification. The only real limitation is whether you want to keep under some exit pupil size. For a young person and under dark skies and with full dark adaption you might want to limit yourself to a 7mm exit pupil and thus 72mm / 7mm ≈ 10X. That would mean an eyepiece with a focal length of 432mm / 10 ≈ 43mm. However, you can use a longer focal length but then the eyepiece exit pupil will grow to larger than 7mm which just means that your eye isn't accepting light from the entire aperture of the scope (but, the image will still be as bright as possible given the limits of your vision).
In terms of the largest true field, that's controlled by the size of the field stop in the eyepiece and that depends largely upon the limiting diameter of the eyepiece tube itself. Tele Vue likes to advertise that their 41mm Panoptic give the largest true field possible in a 2" eyepiece, but that would be true of any eyepiece that has the same size field stop (46mm in the Panoptic). It might even be possible to find a 2" eyepiece that has a slightly larger field stop, but there is an upper limit just because you're using a 2" eyepiece.
So, starting with the 41mm Panoptic we'd have: (field stop diameter / scope focal length) x 57.3 = (46mm / 432mm) x 57.3 ≈ 6.1 degrees.
Note, it's unlikely that the WO Swan 40mm would give 6.5 degrees, since that would mean its field stop would be: (6.5 (degrees) x 432mm) / 57.3 ≈ 49mm
Problem is, a 2" eyepiece has an outside diameter of 50.8mm which means the walls of the tube and any retaining ring to hold the field lens would be just 50.8mm - 49mm = 1.8mm in total thickness (that's the two walls of the eyepiece tube and the retaining ring, so we're talking a faction of a millimeter for each wall and the retaining ring itself, very unlikely).
Eyepieces that offer wider apparent fields of view just allow you to use higher magnifications before you reach the limits imposed by the diameter of their field stop. The higher magnification means a smaller exit pupil, which could be an advantage under some situations.
All that said, Tele Vue does offer some useful tools to select eyepieces, here are some of their references:
Choosing an eyepiece:
Note, you can use the information provided in the links to help guide you in the selection of just about any brand or make of eyepiece, you don't have to buy from Tele Vue.