There isn’t a text limit within WordPress itself, but you may be bumping into memory limits at your webhost.
Using FTP you can try increasing the memory for PHP and WordPress several different ways:
1) You can edit the memory_limit line in your php.ini (if you have access to that file) to 64M:
memory_limit = 64M;
2) If you can’t get to the php.ini file, add this line at the top of your .htaccess file:
php_value memory_limit 64M
3) If that doesn’t work or throws an error, add the line below near the top of your wp-config.php file right after the opening <?php
define('WP_MEMORY_LIMIT', '64M');
If none of those work, your host probably doesn’t allow memory allocation overrides. You’re stuck with what they give.
You can run phpinfo to see what kind of memory allocation you already have. Make a plain text file and call it phpinfo.php and put just this line in it:
<?php phpinfo(); ?>
then put the file in the root of your server, and then go to that file with your web browser. You’l see all the php info you need; scroll down for memory settings. If you get an error, that means your server company won’t allow phpinfo to run for security reasons.