18.6.1.2487.25
XC-SIZE
x-c-size
XCHAR
 
( xchar -- u )

u is the number of pchars used to encode xchar in memory.

Implementation:
: XC-SIZE ( xchar -- n )
   DUP $80 U< IF DROP 1 EXIT THEN \ special case ASCII
   $800 2 >R
   BEGIN 2DUP U>= WHILE 5 LSHIFT R> 1+ >R DUP 0= UNTIL THEN
   2DROP R>
;
Testing:
This test assumes UTF-8 encoding is being used.

HEX
T{      0 XC-SIZE -> 1 }T
T{     7f XC-SIZE -> 1 }T
T{     80 XC-SIZE -> 2 }T
T{    7ff XC-SIZE -> 2 }T
T{    800 XC-SIZE -> 3 }T
T{   ffff XC-SIZE -> 3 }T
T{  10000 XC-SIZE -> 4 }T
T{ 1fffff XC-SIZE -> 4 }T