Will think about it for a bit.  My initial inclination is against
because there have been no other such requests to instrument all the
fundamental data types, because it's not hard to add instrumentation
using your own pure python additions around size-mutating calls, because
it clutters the API, and because it's unclear whether a maximum-size
statistic has much utility.

For deques, should the clear() method zero-out the high-water mark?  How
about the equivalent code where the contents are all popped-off (down to
zero size) before the structure is re-used?