For long-focal-length eyepieces that show a field of view that occupies the entire physical barrel of the eyepiece, a 2-inch diagonal helps. With such eyepieces (most 32mm or longer Plossls, or the new TeleVue 24mm Panoptic, for example) the eyepiece barrel itself serves as the field stop. You don't want anything that is 1.25 inches or narrower anywhere earlier in the light path prior to the eyepiece, since it would cause vignetting. However, for high-magnification eyepieces (with smaller fields of view), there's little or no difference between a 1.25" or a 2" diagonal.

--Dan Johnson