Barry's 100% correct, Chaz, you don't need the extra length on the sides. Beyond adding unnecessary weight, you're blocking air flow as you approach the side walls. Air flow on the front and back isn't necessary, but airflow on the sides is critical. If you're comfortable with a slightly heavier plate, you can give yourself a little extra on the sides to give yourself a larger lateral target, but I wouldn't go too much. If, say, you've got 17.5 front to back, maybe go 18 or 18.5 side to side. Try to leave a 1.5" gap on each side (3" total) for good air flow.
I've recently come up with a new way of measuring the front to back space. Take a piece of paper, set it on the shelf with an an inch or two sitting off the shelf/out the door. Close the door and let the door push in the paper. With the paper pushed in, measure the distance to the shelf and add this to the shelf dimension. Do this a few times, on both sides of the oven, to make sure you're getting the perfect measurement.
Make sure you position the paper in the area where the door protrudes towards the shelf the furthest, as some doors will have windows that curve away from the shelf. Also, when you're closing the door to push the paper, give it a little bit of extra pressure to compress the door's seal.
As you measure the shelf itself, make absolutely certain that you're aware of any lips in the back, as many shelves have a bar running across where your stone will go, preventing the stone from touching the back wall.
By pushing the paper with the door, you're getting every possible fraction of an inch on that back to front dimension. You can NEVER have too big of a target. 17.0625" (17 1/16") is far better than 17". If you get the plate and it ends up being 1/2" away from the door, that's just as tragic, imo, as if the plate were too large and the door didn't close.