300 dpi should be fine. I don't know 'bout sizes as they tend to vary depending on personal preference - the two I have in my wallet are 90mm x 52mm and 83mm x 54mm respectively - about credit card size as a general rule. Paper weight should be at least 160 gsm I reckon, and you're right with CMYK. As for formats, I tend to use PDFs as any printer will accept them, although make sure you check the export settings as different printers may want different compression rates or bleed marks etc.
As for A4 - it's easier to say than 8.5 x 11...
