ntuser\client\usercli.h:
/*
 * Allow an initial maximum of 30000 characters in all edit controls since
 * some apps will run into unsigned problems otherwise.  If apps know about
 * the 64K limit, they can set the limit themselves.
 */
#define MAXTEXT         30000

....

ntuser\client\edecrare.c:

/**************************************************************************** ECCreate AorW
*
* History:
\***************************************************************************/

BOOL ECCreate(
    PED ped,
    LONG windowStyle)
{
....
ped->cchTextMax = MAXTEXT; /* Max # chars we will initially allow */

ограничение специфично к edit control:
ntuser\client\usercli.h:
/*
 * Allow an initial maximum of 30000 characters in all edit controls since
 * some apps will run into unsigned problems otherwise.  If apps know about
 * the 64K limit, they can set the limit themselves.
 */
#define MAXTEXT         30000

....

ntuser\client\edecrare.c:

/***************************************************************************\
* ECCreate AorW
*
* History:
\***************************************************************************/

BOOL ECCreate(
    PED ped,
    LONG windowStyle)
{
....
ped->cchTextMax = MAXTEXT; /* Max # chars we will initially allow */