A wacom tablet is just a computer input device were the user can 'draw' with their pen onto the tablet as though they are drawing onto paper. It allows for a lot more fluidity for the artist that a mouse. Lots of programmes (Photoshop, Illustrator) support Wacom tablet use. Personally I find them a pain to work with unless you are doing basically brush stokes.
As for the rest of your question, I guess the smaller your artist draws his picture the less detail he can get into his work - depending on his style of course - but as far as the entire scanning/repro side goes you shouldn't really lose to much detail due to different sized originals.
If the detail is there in his six inch drawing, it should still be there in your 12 inch enlargement (if the scan is any good - and it all depends how you are printing it) In fact you are more likely to lose detail going from an 18 inch drawing to a 12 inch screenprint rather than the other way around.
If the art is originally created as a vector then it is resolution/size independent so it'll be as detailed whatever the size (unless it's REALLY small) I doubt however that your artist is creating this thing in a vector programme (like illustrator). If he is and he charges more for a large picture then ask his to give you a nice cheap half size image as it will scale up perfectly well
Art to Scanner to Screenprinter Question 
