poster and degree show

mike has asked each of us to work on a poster and websight towords the degree show.

the posters have to be A1
1684 x 2384 ppi - 4 million pixels

the posters will depect the chartoers from the short film i am working on with rob.
i have done the basic sceaches and inking and sorting out colour as we speck.
the problem is since the image is so big. my computer cant handel the pixel size.
as each layer is aded the time it takes to takes to just save goes up.
shading is going to be a nightmare