The design process I used for several of my rock posters.

Posters are really easy and really fun… The basic format and size I use in Photoshop is 13″ x 19″ @ 240dpi. I work in sRGB color because I am primarily designing these for use on the web. I use 240 dpi because that is plenty of data for my in house Epson printers if I need to print them, and 240dpi (vs the traditional 300dpi) keeps the file size manageable.