Your printer uses the DPI  associated with an image file info to detrmine the final printed image size. This can be overridden in most printers to increase or decrease the print size in the print settings dialog. 
For use with a 4 colour dot-on-dot halftone process 300 dpi is about where you have a break-even with most screens in terms of dot size and the colour gain experienced by the paper stock.
You can go lower than this (down to 100 dpi typically) on an inkjet if you can take advantage of error diffusion halftoning. This technique employs a slightly random distribution of the print dots as opposed to the rigid grid set by standard halftoning. This allows for a bit of optimisation giving superior result at lower dpi.
At least that's the way I understand it.

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