As Terence has implied, size and resolution measure different things. Computer image files have a size. It is measured in pixels, and always has a height and a width. The file itself does not have a resolution: the resolution is a measure of information density, and so needs a physical size in order to be calculated. Computer image files are rarely fixed in size (hence "zoom-in", "zoom-out", etc.). The resolution of computer image files varies.
These files do have a data field that can hold a suggested resolution. This is the "DPI" specification found in, among other places, Aperture's Image Export Preset dialog. (Just to further complicated things, "dots per inch" is properly a printing specification, and printer dots are not and do not equal pixels. The information this field contains by convention is pixels per inch, or PPI.) If you want the files you create by exporting your Aperture Images to carry this information, specify it in an Image Export Preset that you apply on export.
Some programs, at some times, use this information (DPI) to determine the display size of your Image. This can (and should) be ignored by almost all users.
Prints, on the other hand (in contrast to files), have fixed physical dimensions, and thus do have a resolution. The resolution is determined by the amount of information (pixels) and the size of the print (by convention, inches high and wide), at the time of printing. It is widely accepted that 300 pixels per inch is more than enough for highly detailed (= high resolution) prints. A skilled printer should be able to make anything with 200 or more PPI look good even examined close up (with the naked eye).
Prints are made to the size specified. The "DPI" listed in the file is ignored.
A workflow for "high resolution" prints is:
- record as much good data as possible (record many pixels, don't compress the recorded data, save data in RAW format)
- maintain your bits-per-pixel until printing (convert RAW to TIF or DNG, avoid JPG)
- in any developing or editing program, maintain as many pixels as possible (in Aperture, export at "Original Size")
- don't expect to print at lower than 200 pixels per inch without noticeable loss of acutance ("acutance" is "perceived resolution").