Im negotiable on price, but given this forum I've seen much more done for much less.
Size: The posters are generally flyers printed on an A4 bit of paper with some A3 posters printed up within the venue, most advertising is done through facebook so as long as the image can be stretched/shrunk without a great deal of distortion I don't really care.
My style in music ranges from trap, bounce and house. Examples:
http://www.soundcloud.com/scndl