To find P-90 dimensions, try searching images.google.com for "P90 pickups", then just browse until you find a drawing that looks like it's got dimensions on it. I find it is better browsing the images than trying to search "normal" google. Here are some drawings i found:
http://www.axiomatic-music.co.uk/acatalog/p90_metric.jpg
They also have specs, but they cost 0.99 pounds:
http://www.axiomatic-music.co.uk/acatalog/...ifications.html
The bottom of this page has a similar pic but in imperial measurements. However I find that those drawings look slightly different at the bottom compared to how I thought real P90's look. Seems like there is more space than usual under the plastic bobbin. I thought they were meant to look more like this:
http://curtisnovak.com/pickups/p-90.shtml
Anyway, unless you're trying to replace previous pickups, you're free to design them the way you want, and I don't see the need to get everything within a fraction of a mm of what someone designed half a century ago...
I've been thinking about making some P-90's myself in the future, but I think I'll just make my own designs based on what I like, rather than copy them down to the last little detail.
Heggis