I used 128-bit as a max. I just don't like some unbounded thing when we know that there is a practical max. I wouldn't call something perfectly fine when getting the number of characters actually displayed on the screen is a chore.
What do you mean by "getting the number of characters actually displayed on the screen"? Do you mean number of glyphs? Number of grapheme clusters? The number of glyphs is dependent on your text rendering system and fonts; even with plain ASCII, you may have multiple codepoints rendered as a single glyph, such as in ligatures like "ffi" in certain fonts. The number of grapheme clusters can be computed by the given algorithm along with some tables of character properties; it's not particularly simple, but trying to represent languages that were originally hand-written on a computer is not simple: http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries