Open up /admin/_edit_form.php

find this bit:

        <?php // ---------------------------- TEXTAREA -------------------------------------
        // Note: the pixel images are here for an IIS layout bug
        ?>
        <div class="edit_area"><img src="img/blank.gif" width="1" height="1" alt="" /><textarea rows="16" cols="40" name="content" id="content" tabindex="4"><?php echo $content ?></textarea><img src="img/blank.gif" width="1" height="1" alt="" /></div>

Change the rows value from 16 to whatever you want. That should make it taller.

Note: edit these core files at your own risk. Back up your original files in case you break something. And remember that the next time you upgrade b2evolution this file will be overwritten and your changes will be lost.

Thanks Personman, it worked great.

knotty