Confirming on R2017a that the second output of legend has the necessary handles e.g.
[~,b] = legend('foo','bar','baz')
6×1 graphics array:
However, they are nested within the Group objects, and they are also a mixture of line and patch objects. An easy way to change the size then is to use findobj() to retrieve all of the handles with the MarkerSize property, and then to set them i.e.