There is a good article on Slim Jims here at DM News:
Slim jims measure up to 6.125 inches by 11.5 inches. The USPS considers them folded self-mailers and booklets. If they weigh 3 ounces or less and are tabbed they are the largest size qualifying for letter-rate postage, which is significantly lower than recently increased Standard Mail flats postage.
Mailers today need to be able to explore all options, and think out of the box regarding their entire mailstream. Slim jims are one of the methods of redesigning your mailpiece to help save on postage.