actually, not necessarily.
There is only 1 dimension possible IF you know the aspect ratio of individual pixels.
Assuming that the pixels are square, and assuming that the 3.5 inch of the N900 and the 3.7 inch of the Droid are accurate (they are possibly rounded up), that would mean that the screen of the Droid would be 8.2% larger than that of the N900.
If the Droid would have the same aspect ratio, then it would be 11.8%, but the screen of the N900 is less "wide", so it is 8.2% .

Again this is only the case if the pixels are square and if the indicated diagonal size is correct.