The shortest XTC song title is "Red" (3 characters), on Go 2, which happens to be the shortest XTC album title.
The longest XTC song title is "You And The Clouds Will Still Be Beautiful" (42 characters including spaces), on Wasp Star (Apple Venus Vol. 2), which happens to be the longest XTC album title.
I was amazed; I had forgotten about "Red", and thought the shortest title would be a three-way tie between Mummer's bonus tracks "Jump", "Toys" and "Gold". Also, for a while there, I thought the longest title was going to be a two-way tie between "When You're Near Me I Have Difficulty" and "Seagulls Screaming Kiss Her, Kiss Her" (both 37 characters, counting spaces and punctuation).