If you’re instructing a designer then it’s up to you to explain (in detail) what it is you expect whether it be dimensions of individual items for your coder or something else.
That should form part of your contract so that you can build the site you want.
Also, remember that, from what you have said, you have asked him to design the ‘look’ of your site. If your site was going to be responsive then multiple designs might be appropriate and thus specific measurements might be less relevant as these might be substituted with %ages.
Even once you have the PSD and you are now coding it…is it really necessary to know that a box is exactly 957px wide in the comp. What if you are designing for 960 px? Wouldn’t you just assume that he meant 960px?
If he gave you a PSD based on 960px but you are coding for 1140px…who’s fault is that?
As I said, you’re apparently paying for the look…not the code…at least from your OP.