problems with my menu

Either you play with your template css file to make the submenu shorter then maybe menu can fit to the screen OR you may reorganise your menu structure to have 5-6 items each submenu.

There is a line in file:
http://www.wooden-workshop.co.uk/wp-content/themes/kingsize/style.css

#navbar li ul li {
padding: 0 0 10px 0;

change it to:

#navbar li ul li {
padding: 0 0 0px 0;

And another line:

#navbar li ul li a {
padding: 12px 0px 8px 30px;

Change it to:

#navbar li ul li a {
padding: 8px 0px 8px 30px;

This will shorten your submenus so that they can fit