I was looking for a bag large enough to take my Samsung R720 17.3" laptop, which is 412 mm long (16 1/4" in real money). I really fancied a Belkin Messenger 17" bag as I have found Belkin bags to be of excellent quality, but customer reviews and the bag info available led me to believe that one of those might be a bit too tight.
The size of the Port Designs bag seemed a safer bet, but it turned out to be pretty useless. It was big enough by a mile, but the laptop flapped about in it without any effective means of properly securing it. There were no pockets in it for all the normal bits and bobs that go with a laptop, in fact the only useful fitments were loops to stick a couple of pens in.
There was no proper storage for your mains adapter, cables, mouse etc, in fact all it had apart from the laptop pocket, were spaces in which to slip documents, magazines, or a very slim bacon butty.
To cap it all, the padding was minimal and the construction flimsy, so DON'T BUY IT!
I sent it back using Amazon's superb returns service and ordered the Belkin bag I really fancied. This is a lovely job and fortunately was just large enough to take my laptop, but don't expect it to take anything bigger.