It's a thin plastic board, usually about 7X10 inches. Often with drawings of various manga or anime (you can get all sorts of non-anime as well). They're meant to be used as a flat surface to put paper over and draw, but they're more usually used as collectors items nowadays.
I'm selling this shitajiki from <insert anime here> for <insert price here>.
Something that happens on such a huge scale it could be biblical- ie in the bible. So when Jesus walked on water or performed one of other many miricles they are considered on bibical proportions, after all they're in the bible.