Some modern fridges will stand proud of the kitchen units, whilst older ones will sit further back into the recess. If you were to open the fridge door, and then measure the combined width, you'd find it was about 650mm however, as a fridge normally requires at least a 40mm gap around it for air circulation, you need to take all these things into consideration. So, you'd actually be looking for a gap of about 700mm, and that would then mean you want a 50mm filling piece aither side of the bridgine unit, but as they tend to get screwed to the side units, you need a couple of packing pieces on either side to secure the bridging unit to the packers and then to the side units.
Although it's not a cheap way of doing it, I use a end panel to make up the filling pieces, and that gives it a much better match than using plinth. Alternatively, as the bridging unit will cover the top, you could swipe a bit off the back at the top for the filler pieces.