Material Broswer: Texture Size settings...
Click the chain to toggle it from unbroken/broken.
With an unbroken chain the edited sizes change in proportion, e.g. make one twice as big the other one changes to twice its size to match.
With a broken chain you can change edit one size and the other size will remain as it is.
Making the chain unbroken again will re-link them ,and once again they change in proportion; although at this point you may well have got two unequal sizes they will still be changing 'proportionally' - e.g. twice the size...