You can blow up a 6 MP image to the same size as an 18 MP image. Also, in all likelihood, you have the wrong impression on how these things work.
Dividing pixels by resolution is quite pointless: the resolution you need depends on so many different factors that it's impossible to give just one rule you should stick to. There are a few guidelines in my opinion:
(1) If the picture is very good, very few people will criticize that the `resolution is too low.' Never happened to me. I've had 4 MP blown up to 45x30 cm^2, 16 MP to 90x60 cm^2, etc.
If you are a commercial photographer, this may be important, but from the looks of it, you're not.
(2) The size of the print depends most importantly on the content. This is just a gut feeling thing: some photos need to be printed big while others don't have to be or shouldn't. Since it is a matter of taste, there are no rules, do as you like.
(3) Some pictures are grainy or benefit from having grain added to them. The 16 MP image, for instance, was a professional scan of a BW negative. The scan was so good, it captured the grain of the film.*
(4) The texture of the material you print on is crucial: if you are printing on canvas, for instance, you don't need as much resolution: the material is rather rough, 75 dpi is probably plenty for this material. On the opposite end of the spectrum is glossy photo paper. The choice of material to print on depends on your taste and your photo.
(5) The larger a print, the larger the viewing distance. Hence, you don't necessarily need more resolution if you have bigger prints.
Currently, ~12 MP seems to be the sweet spot of APS-C-sized sensors. If you want to use the extra resolution of an 18 MP sensor, you need good lenses. If you think of buying a camera with a kit lens, I say forget about it. Also, image size scales as the square root of the megapixels: even though an 18 MP camera has 50 % more pixel, it allows you to print images 22 % larger on each side (at the same resolution). Even doubling the pixel count from 6 MP to 12 MP allows you to have ~40 % larger prints.
* Even though it was an ISO 125 film, it was very grainy. The amount of grain also depends on the way the film was developed, for instance.