nick for something the size your wanting to print. you're gonna way 300-350dpi.. Also, depending on how creative you want the image to be, something that size in a TIFF is gonna be pretty freaking big man.. Several hundred megs depending.

When i print 16X20" photos, those images are around 300megs. Or so... Granted those are photos, with a lot more going on.. But, you get the idea.