Getting constellation size as a variable

When using the Constellation Object block in Gnu Radio Companion how can
one obtain the size of the constellation, M, or the number of bits per
symbol? This value is needed so other blocks, e.g. Pack k Bits, can be
adjusted accordingly when the constellation size is changed.

A simple example would be "Random Source" producing a binary output,
followed by "Pack k Bits" to create symbols, then "Constellation
Encoder" to convert the symbols to complex-envelope symbol values.
Currently when the constellation size is changed it is necessary to also
change an additional variable, the number of bits per symbol (which is
needed by the Pack k Bits block). I'd like to be able to get this value
from the constellation object.