I have a phat PS3 on an Ikea floating shelf just like this one, & it's got no chance lying flat, & that's before you plug the cables in the back. Mines upright, at about 45 degrees, & it still takes up the the whole shelf front to back.
Looks good, but you can't see the hard drive light, so when games tell you not to turn it off when the lights flashing I'm kinda stumped.
He just means make a cardboard template on which to gauge how your PS3 will fit I think.
Why dont you go for the lack shelve and clad it in good quality Oak flooring panels or similar, I reckon this will give you that all important few extra centimetres of width. I'd double up the wall anchorage for the additional weight (ensure you screw into a stud).
The Oak flooring is just the means to get a pretty wood finish. And I simply mean cut the correct lengths and nail/glue them on top, bottom and sides of the lack shelf. With some sanding a good finish can be obtained. But you will need to strengthen the floating shelf prongs. If it is just a PS3 to sit on it, then heavier duty wall anchors (instead of smaller screws) will do.
I see you have posted in the DIY HC section a floating shelf post. They have all followed the tutorial on the first page with some excellent success. If you are confident in DIY then its a great idea. You could even improve their effort by using a planks of wood stained to your requirements.