I made a file in Illustrator about 900pixels in size and then opened it up into Photoshop, but the size of the image on-screen was considerably smaller. When I zoom in the Photoshop image so it's close to the Illustrator on-screen size it is pixelated. Something is wrong with Photoshop's rulers. Please help!
Also, when creating a new document, say for example 300mm x 150mm at 72dpi the onscreen size (at 100%) is not 300mm x 150mm. It is about a third of the size.
There is nothing wrong. You simply don't seem to understand all this DPI and pixels vs. physical units stuff. Your monitor has a different DPI than the assumed 72 DPI and as far as Illustrator is concerned, it never works in absolute pixels. It's pixels are derived from the document DPI settings. Well, whatever. You should read up on this stuff.
Do you have any references for reading that I could look up? Cheers.