Measure the space you would like to fill with your new splashback, and then deduct 2mm from any edge that will touch a surface.
For example, if you would like your new splashback to fit between 2 kitchen units, deduct 2mm from the left edge and 2mm from the right. If it's to fit between your worktop and underside of extractor fan, deduct 2mm from the bottom and 2mm from the top. Please take care when measuring as toughened glass can not be altered once produced.