I don't have a ruler handy, but they're almost the size of two DVD cases placed side by side, and then turned on their side. They actually come folded in half, shrinkwrapped to the back of the DVD case, with the outer cardboard sleeve slid over that (the posters are actually shrinkwrapped in their own little package, with a piece of cardboard to help keep them flat, and that package is then shrinkwrapped to the back of the DVD).
They're not very big then, obviously (hence "mini poster), but still nice, though I can see how the lack of a proper
Indiana Jones and the Temple of Doom poster (or the wrong title on one of the
Raiders of the Lost Ark one, if you prefer to put it that way) could be a deal-killer for some (it almost was for me).
It's kind of particularly irksome there's no
Temple poster, though, since one of the earliest promotional images for Indy LEGO sets was based on the Bruce Wolfe art for the original
Temple style "A" poster, as seen in promo images from a dealer catalog seen in some of the posts (like
this one) from fellow Raver fixer79 (thanks, fixer!):