There are some general rules you can use to determine the "maximum" (I use that term loosely) print size. Keep in mind that the quality of a print is often more dependent on what is being printed than its size in megapixels, and even if your image size is not dense enough to mathematically fit onto a certain page size, you can still blow most images up pretty large without significant or noticable loss in quality.

Anyway, as a general formula:

$$ \begin{align} \frac{\text{width}\,[pixels]}{\text{print resolution}\,[px\ per\ inch]} &= \text{print width}\,[inches] \\ \frac{\text{height}\,[pixels]}{\text{print resolution}\,[px\ per\ inch]} &= \text{print height}\,[inches] \\ \end{align} $$

Screen/camera pixels do not directly translate to print pixels (PPI, pixels per inch), so you need to translate your image pixel size into a print size by dividing your screen pixel sizes by the PPI of your print. Assuming you have an 8mp camera, your image sizes are likely to be around 3200 x 2400 or so. If you print at the native "photo quality" PPI of an Epson printer of 300ppi, you would end up with the following:

$$ \begin{align} 3200\text{ px}\,/\,300\text{ ppi} &= 10.7'' \\ 2400\text{ px}\,/\,300\text{ ppi} &= 8'' \end{align} $$

That boils down to an 8.5" × 11" print at full resolution without any loss of quality or resolution in the translation between screen and print. However, this is not a general rule for print quality... it is simply a rule for ascertaining the lossless print size for a given image size. You could still blow up your 8 MP photo and print it at 11×16, or 13×19, and still have a good print.

You can use this formula in reverse as well, to determine the screen pixel size your image would need to be to be for a particular paper size:

$$ \begin{align} \text{print width}\,[inches] \times \text{print resolution}\,[px\ per\ inch] &= \text{width}\,[pixels] \\ \text{print height}\,[inches] \times \text{print resolution}\,[px\ per\ inch] &= \text{height}\,[pixels] \end{align} $$

Using this, you could determine what size image you would need for a 2×3 inch print:

$$ \begin{align} 2'' \times 300\text{ ppi} &= 600\text{ pixels} \\ 3'' \times 300\text{ ppi} &= 900\text{ pixels} \end{align} $$

There are other things you can do with your image to improve its quality when printed. Sharpening an image before printing, assuming you do so with the proper care, can improve the quality of your printed copies. It is also possible to use image editing software, such as Photoshop or QImage, to digitally blow up your image to a higher resolution, perform some post process sharpening, and print at even larger sizes.

Beyond this relatively simple explanation, the discussion of how to translate a photo image to its printed paper size is a relatively complex topic. It is possible to print at a variety of PPI, from as low as 100 to as high as 480 or higher. The PPI you choose to print at, the size of your image, and the quality of the algorithms you may have used to scale your image up or down, the texture, brightness, thickness, and color of your paper, and the kind of printer all determine how good a photo looks when printed.