Lots of good info here:
http://televue.com/engine/TV3b_page....212&plain=TRUE
http://televue.com/engine/TV3b_page....n=Advice&id=79
http://televue.com/engine/TV3b_page....=Advice&id=230
http://televue.com/engine/TV3b_page....n=Advice&id=80
The Moon is 0.5 degrees in diameter (+/- 0.05 approx.). Select an eyepiece that gives approx. 0.65-0.85 degrees true field of view (TFOV) for nice framing of the subject. The Moon will fit inside 0.55 deg. but looks better when there is a bit of space around.
TFOV = Field Stop Diameter of Eyepiece x 57.3 / focal length of telescope.
The following can also be use to calculate TFOV and is not 100% accurate due to distortion but close enough.
TFOV = AFOV / Magnification
Magnification = Focal length of Telescope / Focal Length of Eyepiece
See also
http://televue.com/engine/TV3b_page.asp?id=214 and look in Field Stop column and apply to formula above or simply click on first link above.
Tele Vue publish their FSD data whereas not all others do.
The AFOV determines the 'width of the window' that you look through. You can see the same amount of Moon/Sky through separate eyepieces at different magnifications. The one with the larger AFOV makes the object appear larger/closer and you have to look around to take it all in. Eye relief too comes into play here and determines viewing comfort to some degree.
The other term you will hear is Exit Pupil which relates to the brightness of the image.
Exit Pupil = Eyepiece Focal Length / Telescope Focal Ratio
also
Exit Pupil = Telescope Aperture / Magnification
Select eyepiece for an Exit Pupil range of 0.5mm to 7mm typically although the lower and upper limits can change on telescope type and observer. Don't worry about exceeding the upper limit on refractors and too on Reflectors (with obstruction) as the impact of the obstruction is often overstated but be wary of it for the latter. At the lower limit eye floaters come into play and on the Moon in particular they are a nuisance. The image too starts getting dim but the type of object you are observing also comes into play. If a Planet image brightness and contrast will be low but for double stars this is not so much a problem.
Whatever you choose, enjoy!