In digital imaging devices megapixels define the resolution range when the number of pixels is equal to, or greater than 1,000,000 pixels.
This is found by multiplying the active horizontal by active vertical pixels. eg SXGA is 1280 x 1024, or 1,310,720 pixels.
This file saved (or open in photoshop) as an uncompressed tiff file as RGB @ 72 dpi will be 3.75 MegaBytes in size.
This file saved (or open in photoshop) as an uncompressed tiff file as CMYK @ 72 dpi will be 5 MegaBytes in size.
This file saved as a compressed jpg file (either CMYK or RGB @ 72 dpi) will be vary in size but may be around 1 MegaByte (depending on the compression level and the type of image).