About 1.7 inches (lol) it's personal preference, that's one of those how long is a piece of string questions really. You need to decide for yourself as I prefer smaller machines for moving around but some prefer 17"+ monsters and then some people insist on a desktop for gaming.
Is there a noticeable difference between 15.6 and 17.3 screen sizes or isn’t it worth the extra £’s?
It's It's much better GPU and the screen is 144Hz...
I get that it has an i7 rather than the more common i5, but £1250 for a 1060 machine is very expensive. If you are buying this strictly as a gaming laptop (which is what it is) then the benefits of getting an i7 over an i5 frame rates wise will pale in comparison compared to upgrading to an 1070 instead, which is what I would be looking at for this sort of money. There are sub £800 laptops with 1060's. The only reason I see to get the i7 over the £400+ saving on an i5 one is if you want to stream your games. For me, this isn't a great deal.
The below for £1,500 with a slightly better GPU reached 250 degrees https://www.hotukdeals.com/deals/asus-rog-strix-gl703gs-144-hz-i7-8750h-gtx1070-256gb-149997-amazon-3031739#comment-35001775