FWIW, I just received a pair of Neumann WS89 windscreens for my AKG C414B-XLS. After measuring the XLS at 48mm wide x 37mm, I was torn between the WS89 (~46mm inner diameter) and WS87 (~56mm inner diameter), but went with the WS89 since I was concerned the WS87 would prove too large.
And...the WS89 fit!
A snug fit, that is. On first application, while pushing the w/s down onto the mic, the leading edge of the w/s got stuck mid-way down the grill. So while pushing, I gently pulled the w/s down onto the mic, rotating the mic and gently tugging in small increments for each side of the XLS. I let the w/s sit for a while on the mics, then took them off and let them sit again until the w/s reverted to its original shape. (The w/s internal hole is round, while the XLS are boxy.) On second application, I pushed the w/s down til the leading edge caught the main body of the mic, where the grill meets the body. I then gently pushed/pulled it on the rest of the way. I don't know if they'll loosen up enough to push them on all the way without switching to gentle pulling at the grill/mic body intersection, but it's easy to do, doesn't take very long, and they're secure.
In hindsight, I neglected to take into account the diagonal "diameter" of the XLS. Based on my measurements of 48mm wide x 37mm deep - and if I remember the Pythagorean Theorem correctly - that would make their diagonal "diameter" ~60mm. So, maybe I should have gone with the WS87, I wondered? But I don't think so. The WS87 inner diameter is ~56mm. The XLS, given its boxy shape, would only contact the w/s interior at the mic's corners, leaving space on the front/back and sides. Space into which wind might flow in breezy conditions. So while I didn't think it completely through up front, it seems I made the proper choice.
2007-02-14 - Edited to add:Added some pics. As you can see in pic 04, the WS89's leading edge catches a bit on the squarish lip where the 414's mic body meets the grill. A gentle tug in each corner slots the windscreen all the way on.
01 - Side by side
02 - Side by side, end view (mic bottom)
03 - Side by side, end view #2 (mic top)
04 - Partially on, gentle tug
05 - Full on