What type of breaker for this box
I installed this same surge protector in my 50-year-old GE panel with a 50 A double pole compact breaker (1 inch wide).*
Note that this must be installed astride two slots to get both legs. The breaker and the panel together have a mechanical incompatibility preventing it from snapping into a single slot.
This means the compact two pole breaker cannot be installed at the very top, but must be shifted 1/2 slot (1/2 inch) down. I don't think this reduces performance since this device is in parallel with all the other breakers in the panel. You can put in a 1/2 inch breaker in the half slot above and that circuit would still be protected.
If you would install a full size 2 inch wide 2 pole breaker I think it could be installed at the top, but then it would occupy twice as much space in the panel.
You do not need heavy wires to connect this surge protector, just use the leads that are attached to the case of the surge protector.
*I  think I used a GE THQP250.
EDIT My experience is limited to my very old panel. Pay attention to the statements of others about where the 2-pole 50 A breaker will fit. If there is resistance, stop and try another pair of slots. I do not think the breaker for the surge protector needs to be at the top.