The answer is actually right there on your data sheet. The circles on the right are an end view of the device. The smaller of those 2 circles represents the diameter that would be going through the drilled hole you are asking about. You can see that they have indicated it as 7.4 (and i'm assuming this datasheet is in millimeters?) So, if you make a hole the same diameter as that, you are going to get a pretty snug fit.
There is a down side to that. First, its actually tricky to put it in, cause the body and the hole have to be perfect with regards to tolerance on the measurement, and if they are, you still have the issue that if the hole is not perfectly perpendicular to the surface, the top bezel may not sit flush with the surface, rather one side will have a small gap.
You can avoid both of these by going with a loose fit, and make that hole just a tiny bit bigger. The device will go into the hole easier, and will have a little wiggle room for the front to go flush when you put the nut on.
On the other extreme, if you make the hole too big (in this case, 9.5) your device will fall right through the hole.
All right there on the data sheet.
You could also look it up, but they are nice enough to give us that data sheet so we don't need to, cause they already did... so if everybody goes looking these things up on their own, the guy they pay to look it up looses his job. :)