Option A: Fix all parts correctly by putting in correct dimensions, it won't take as long as you think. Also, all your mates to move arm parts around will still be there.
Option B: New Part -> Pick default template -> Exit sketch mode if default is to start from there, Under Model Tab choose Derive -> Pick assembly/or Part to bring in and on far right tab there should be an option to scale the part under the Options tab.
You can them save and import each new arm back in.
You most likely will find it more difficult to re-pose arms as they are now another level down in the assembly.