As I received the buttons, I was able to not only get electrical information but also dimensions. I updated the layout:

The 50 buttons easily fit in a 18×18cm square so 20×20cm is a safe dimension for this panel.

Remember that it's only the instruction, and more things are done in other panels :

• The disassembly panel shows the instruction being sent to the core (it's the reverse of the ASM panel)
• The control panel selects the source of the instruction, generates the clock, sends the clock pulses to the  core (one-shot, several ones or continuously)
• The debug panel has several event counters that can stop the core...
• More panels display the inner state of the core