/usr/local/directadmin/data/skins/[yourthem eg: Capri]/admin/log_viewer.html
<tr><th>|LANG_NUM_LINES|</th><td><input type=text name=lines value="200" class="combot" size=7>
You'd better check this http://help.directadmin.com/item.php?id=295 if you don't want to modify the file every time after you upgrade Directadmin.
admin/log_viewer.html
admin/log_viewer_modified.html
lang/en/admin/log_viewer.html
lang/en/admin/log_viewer_modified.html