You will have to edit the userblock.php in mod_k2_login. Locate <a class="modal" .... </a> and change the width of the modal window.
If you want to change it also for the Edit links and to Add a new item in this category you will have to make that change also to the cateogry.php, item.php, category_item.php etc. Locate itemEditLink, catItemEditLink, catItemAddLink.