Ever since we started, we've stocked full size and half size prototyping boards. The standard sizes allow the boards to be stacked together, which is why we called ourselves protostack.
This month we are stocking a new board, which is approximately a quarter of the size of the full size one. Instead of calling it the "quarter size board", we've renamed all the various sizes to Small, Medium and Large.
The diagram below shows the 3 board sizes in relation to each other. As you can see, all 3 board sizes are stackable with the vertical blue lines showing how the mounting holes line up.
The small boards can very easy be stacked together as shown below.
When stacking the small board with one of the medium or large boards, you will need to support one end with a stacking header or something similar. This is shown in the photo below.