Here's an approximate screen measurement. Note that it is not official.
There is no doubt the main feature of the HTC Touch HD is a big screen in a small form factor. Unfortunately we know who big the device is, but have no idea what the exact size of the screen is. Fortunately a bit of image manipulation will soon reveal all.
First we perspective-correct and crop the best front-on picture.
Then we resize the picture so it is calibrated to the mm ruler, so the known width of 63mm is also 63mm on the ruler.
We then use this calibrated picture to measure the screen size. The screen is 53mm wide…
… and 85mm long.
Using a similarly calibrated inch rule, we can easily see the screen is around 3.8 inches diagonal, but not 3.5 or 4.
So there we have it - 3.8 inches - much bigger than the iPhone 3G but in a smaller package. Congrats HTC.