Resolution is the amount of detail captured in any given image. It is determined by the number of pixels present in the image at the point of original capture.
DPI, or more accurately PPI (pixels per inch), is by itself a useless number. It is the product of PPI and document size which determines file size, or the amount of detail which can be resolved.
Upsizing a file is not recommended – it does not create greater resolution (remember, you’ve already established the file size at the point of capture). It simply increases your file size via interpolation. Interpolation, driven by sophisticated mathematical algorithms, creates more pixels by estimating what those new pixels should be, and adding those new pixels thereby increasing file size.
For most portrait applications, a file size of between 15 megs and 25 megs will serve very satisfactorily, even for prints up to 30×40. Most programs display the file size in one place or another – you may have to look for it. Photoshop displays the file size at the bottom left of each opened image.