All the dinettes I've used essentially worked the same regardless of width. Most are around 6-ft long (more or less a few inches), with the bottom cushions around 23 to 24 inches wide (or is it length?) and the back cushions around 12 to 13 inches tall in seating position (part of what makes most dinettes uncomfortable to sit for long periods).
The four cushions when placed as shown in attached picture should fit tight, otherwise cracks between cushions would be uncomfortable to sleep on.
If yours is like this but the four cushions don't fill the length of the dinette in a tight pack, I would question if you have the correct cushions for the dinette length.
And for what it's worth, I prefer arranging cushions differently than in picture to make bed more comfortable to sleep on.