Their size is 300 x 300 pixels. If you were scanning at 300 dpi, a 1 1/2" chip should be about 450 x 450 pixels, and would be closer to 4 1/2 inch size on most PC screens. An easy way to remember is that when scanned at 100 dpi, most objects will display approximately full size on the PC screen (at least on my screens).

Can you show a screen-capture of the screen in the scanning software where you set the resolution? I think you may be setting the wrong parameter. It's also important not to set the JPEG compression or "quality" at too low a number, or it will appear with "jaggies" on the PC screen.