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In order to properly size an image for print, you need to know what size it's going to print at and what line screen the magazine prints with and to a certain extent, the type of paper you're printing on. The traditional rule of thumb is to have your file be twice the line screen ruling in resolution at final print size - meaning that if your image is going to print a full 8-1/2x11 page at 200 line screen, then, ideally, you'd want your file to be 8-1/2 x 11 @ 400 ppi. High quality, whatever that means, is usually printed on a premium coated paper using 175-200 line screen. U.S based magazines usually, but not always conform loosely to SWOP or FOGRA web press standards, and spec total ink limits between 280 and 320 percent. Whether you want to leave the CMYK conversion up to the magazine is a whole different topic, but if you have little or no experience in that department, it's better that you leave it alone.
The double the line screen resolution guideline is only a rough guideline, and depending on image content, you may want to go either up or down. Usually you can safely get away with less than double, but with images that might be prone to moiré on press (different from on screen), going with an even higher resolution in your file can actually soften the images in print and help to head off a potential moiré.
The main thing is to not go too low in resolution or you'll be looking at stairstepping of diagonals and pixelation in print - usually not a great thing. After a few images in print, you'll start to get an intuitive feel for what works and what doesn't.