how to control string length for full-width and half-width characters

[Russell]

i want to do a software for japan.
it has a reqirment to control the string lenth. in a textbox.
if the maxlength is 2, then the textbox can be
input 2 "full-width" characters or 4 "half-width" characters .

if i set the control's maxlenth is 2, it can be only
input 2 characters whatever "full-width" or
"half-width" is.

how can i implement it?

thanks!