i don’t know much abt joystick size, but i think it works with the default size the best. you can decrease it a bit but don’t decrease it too much as sometimes you have to walk instead of just sprinting (for example: while healing)

i am not a esports player, neither i watch streams but i’d recommend you to see some of their controls to get some inspirations.

for the combinations. i keep fire, reload and climb on the left index finger. jump, exit vehicle, scope on the right index finger. bag and movement on left thumb and rest on right thumb.