Photoshop file size

The size is determined by the resolution x the dimensions of your document x the color space. So a 300 dpi image will be over 4 times bigger than a 72 dpi image, a CMYK image will be four times bigger than a Grayscale image and 25% bigger than RGB. The Help Center at Best Printing Online has an excellent article on resolution and it's affect on image quality

Web graphics are usually done at 72 dpi RGB which is the resolution and color space of most monitors. Print requires resolutions up to 300 dpi in CMYK mode, which is 1.5 - 2 times the screen line ruling on the plates used for offset printing. For signs and specialized graphics, check with your vendor about their file resolution, color space and format requirements

Tiff format will not add any file size to your image, but will not compress it either, however, each layer you add will add to the size of the file. JPEG format will reduce the file size but it is a lossy compression scheme, meaning, the quality of the image will suffer each time you open and save in this format. When you open a JPEG file it expands to it's tiff file size, so it won't save you anything just working on your desktop or server.

I usually save original images as PSD or Flattened TIFF format. I will make a JPEG copy of an image to upload or e-mail.