Dimensions
3.5×4 km (2.2×2.5 mi)
Source: http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko
As for how to convince her, I just don’t know.
Maybe one way is to suggest to her that there isn’t any reason to fake it.
Actually, that is a very good point!
gee that thing is 2.5 miles across! Big!