Image Sensor & Print Size Ratios.

Image Sensor Sizes:

Roll film cameras of the 35mm variety had an image capture size of 36mm x 24mm and is now the size considered as full frame in digital sensors. The most common print size to the average photographer was 6" x 4" which happened to be the same length to width ratio as the image captured onto the film, (3 units by 2 units within reason).

Compare this to the Four Thirds System Sensor, (as used by Olympus) the sensor size is 17.3mm x 13.0mm. This returns a ratio of 4 units by 3 units, (hence the name Four Thirds).
The nearest photographic paper print size that uses the same ratio is 6" x 8", (obviously rotated to be 8" x 6" when printing takes place). Any other standard sizes would cause some sort of cropping of the image or cause the image to have uneven borders.

Cropping:
Take an image that was captured using my Olympus E-510 DSLR camera. This returns a ratio of 4 units by 3 units as previously mentioned and will produce a full ratio print if 6 " x 8 " paper is used. If however 5" x 7" paper was used cropping would occur on the width making it seem thinner. Using 6" x 4" paper crops the width so that even more of the picture is lost!

Six Selected Sensor Size Scaled Representations:

The other alternative to cropping the image is to have uneven borders where the image is resized rather than cropped to fit the given paper size.

In Conclusion:
To get the taken image from your camera onto print you need to know the ratio of the sensor for your particular camera if you plan to use a paper size that will take the full size image without cropping or resizing to accomodate a new aspect ratio.
For example the APS-C (Canon) sensor, (22.2mm X 14.8mm)returns an apect ratio of 2:3 (0.67) so using the chart above the ideal paper size to print to would be 4" x 6" assuming no cropping or resizing was required.
However the choice of paper available for given standard sizes is somewhat limited especially if you wanted a larger print than the size available. In this instance the option of resizing or cropping to attain the larger print is down to individual choice.
Some photographic development shops will do a custom size print but they tend to be rather expensive.
There are larger standard paper sizes available, (for example 10" × 12", 10" × 15", 11" × 14", 11" × 17" & 12" × 15") but again these tend to be expensive.