You can edit the resource_add template and increase the number of characters permitted from 100.
<input type="text" name="tag_line" value="{$resource.tag_line}" class="textCtrl" id="ctrl_tag_line" maxlength="100" />
However, you will also need to edit the xf_resource table and increase the tag_line field value from varchar(100) to match.
There may also be changes required in the datawriter too.
In short, it's not a simple change.