There are several patterns on Ravelry that will help give you an idea of size and maybe pattern for a lap throw. There's quite a variety of sizes (from 27x38" to 48x48" with lots of sizes in betwen) so it will be a matter of your preference. The number of stitches to cast on will depend on the yarn and needle size that you're using and also the pattern stitch.
its just a basket weave pattern ,but not too difficult
wow that actually sounds pretty
I dream of a world where there is kindness, not hate. Even loners yarn
✿•*¨`*•. (¯`v´¯) (¯`v´¯) .•*¨`*•✿
. . . ✿•*¨`*•.¸(¯`v´¯)¸.•´*¨`*•✿ . . .
I like basket weave patterns, they're pretty. I think the size 8 needles might be a bit large for the DK yarn. Try a swatch and see what you think. You'll need to swatch to get your gauge and determine how wide 78 stitches will be.