CREATE TYPE for case insensitive text and varchar

Hallo.

i just coded a new datatype named "cistring", that can be
used in every function that accepts text or varchar, and has
the great ability to be ordered case insensitive in ORDER BY-
Clauses and Indices. The problem is, that its coded like the
text-type, so it is variable length completly.

I miss the possibility to code something like a userdifined
varchar(n). I don't know how to create an input function,
that accepts the size of the type, or a function, how to declare
such types at all. I think, somethink like a userdefined
numeric(n,m) isn't possible, too, or am I wrong?

Thank you for all replies,
Daniel Migowski