The good news
There is a matcher that does exactly what you want in the current master branch of the JavaHamcrest project.
You can call it like so:
assertThat(mapMap, aMapWithSize(1));
And the bad news
Unfortunately this matcher is not in the latest release of Hamcrest (1.3).
[Update] And finally the very good news
The aforementioned matcher is included in the newly released version 2.1.