Please note that the above image is 1 simple image I did for client. The others are more scenic. I know for this one image the pixel size may not matter but it will matter on the more filled images.
This is much simpler than you think.
An image file is just pixels, nothing else. A camera sensor delivers a certain number of them. That's what you have to work with.
To print it, you naturally need to decide how big you want the print. Now the question is: at this size, will there still be enough pixel density to yield a good quality image?
To quantify that, you take the number of pixels available, and divide by the required print size in inches. This gives you a very significant number: pixels per inch (ppi). Also commonly referred to as resolution.
The consensus is that for standard printing, the ppi number should be around 240 or higher. So that limits the size according to how many pixels you have. If you followed me so far, you now realize that you can do the math. You can use this simple ppi equation to see how many pixels you need for a certain print size. Or you can let Photoshop do it for you in the Image Size dialog.
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As for file size, in Megabytes, it's already been said: that's not image size, that's data size, with or without data compression. The jpeg algorithm is an extremely effective data cruncher. It can squeeze a file down to 1 - 5 % of original size, at the same pixel dimensions - but a somewhat significant loss of quality and integrity. Don't use jpeg unless you really have to.
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