I don't think you can give a single definitive answer to how many pixels there are in a slide; however I hope to give you a good intuitive understanding. Below there are three images that should give you a good idea of how much data there is in a slide.
The first is a small portion, approximately 1.4mm x 1.1mm (130x110 pixels), of a 35mm slide shot on Fuji Velvia and scanned at 2438dpi using a Minolta Dimage Dual scanner. This small portion of the scan was then enlarged 500% in Photoshop using the nearest neighbor algorithm so it would be easier to view. The second is a scan of the same slide using a Polaroid SprintScan 4000 at 4000dpi. It has been enlarged 275% using the same algorithm as the first. The third image is a photograph of the same region of the slide taken using a microscope and then scanned at 2438 DPI using the Minolta Dimage Dual. The third picture was then cropped to approximately match the first and reduced to 1/4 its original resolution. In the third photo you can clearly see the grain from the photo and there is not much point in scanning past the point where you see the individual grains (or more correctly dye clouds).
The original photograph (a portion of section C) was shot using an ElanIIE on a tripod with mirror lockup. The lens used was a Canon EOS 20-35 2.8L set at 28mm and F8.
4000DPI (Polaroid SprintScan 4000)
click on image to see a larger version
Microphotograph of the slide
original 2438DPI image on the left, original 4000DPI image on the right
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