Unless a printer cannot accommodate an unusually large image file, there is generally no reason to downsize an image taken on a typical digital camera for printing. Digital photo printers can print at such a high resolution, that most can print upwards of around a 10 megapixel image at full resolution on one square inch of paper. For example, my printer, the PixmaPro 9000 mark II can print 4800*2400 dpi. That equals 11.52 megapixels in one square inch, at full resolution. As a side note, 4800*2400 dpi is nothing special...Canon's $99 iP4920 digital photo printer can print at 9600*2400 dpi, or 23.04 megapixels per inch. So if you own a new 5D Mark III (22.3 mpx), you can print a full resolution image, smaller than 1 square inch on the iP4920, and not lose a single pixel of resolution.
By downsizing your image, you are unnecessarily removing pixels which most modern printers can easily accommodate, removing resolution and hence, detail from your printed images.