Battery size for '79 635 Euro

[gt40mk2]

Hi - Don't know much about batteries, but I know the one in my car is too small for the tray so my mechanic rigged a piece of wood and bungee to keep it secure.

Can anyone tell me the size required for a '79 Euro E24? (635 csi)

Thanks.

[Bert Poliakoff]

I just bough a group 47 for my 81 euro