If you're confused by the examples given [for the approximate size], that's understandable, but the examples don't change the analysis. That's like saying "All of the countries in the example are in the northern hemisphere, so the analysis is flawed."
We know what a country is. And there's very (very, very, trust me I'm an engineer) little chance we'll find new countries with characteristics unknown to us...
Thus, the statistics are purely for the discussion of "what should other sentient life look like?"
Finally, it's ridiculous to complain in a discussion about aliens "that we can't begin to imagine what type of life form constitutes an alien." You have to make some fundamental assumptions. For instance, we should be able to assume that they obey the physical laws of the universe.
Anything else puts you in a totally unscientific world of discussion [your claims are no longer falsifiable], and that's not one I care to participate in.
I don't think anyone is assuming otherwise... all I see is statistics and sampling being argued. You'll have to endure my wall of text, though, sorry XD.
Let's take the planets again. We don't know the size of every single planet in the galaxy (i.e. the planet size histogram), so we need to make a guess. So we've been using Kepler's exoplanet observations, knowledge of planetary geology, etc. to fit a model, and it's well developed (http://exoplanetsdigest.com/2014/07/25/exoplanet-statistics-...). The size distribution is quite well understood, its bounds and modes are well defined. It's unlikely that our best guess of what the average size of exoplanets is will change drastically the more we know about planets. That is why the article's first conclusion is legitimate.
Now back to (sentient) species, and my original point. The definition of life in the SETI context has to be VERY wide. It has to encompass any scenario for a species that we might consider "intelligent" -- and not just little green men. It might have (some would say, inevitably) evolved beyond the biological, and still be considered sentient. After all, life on Earth has only existed for ~4By, compared to ~13By for the Milky Way. As a base for extrapolation, just think how different to humans extreme life on Earth is. (http://www.livescience.com/13377-extremophiles-world-weirdes...) In any case, there are bound to be some weird-as-shit species out there, whose composition still obeys the laws of physics. In other words, there is almost certainly many statistical modes of life out there that are, or can become, sentient.
With that in mind, there's a very good chance that the overall size distribution of (sentient) species does not match the one the author used (that of vertebrates only). In statistics-ese: if the distribution is multimodal, the average of our unimodal sample is not a good guess as to what the true average really is.
Maybe we can agree upon that?
But, when considering life, if applying the Big Alien logic, I should expect to be a single bacteria. And if not that, then, say, some insect.
But, instead, I'm a human. Or a mammal. Or a vertebrate. Any of these very fundamental descriptions of myself already put me in the SMALL group not the LARGE group when classifying the population of all life on this planet.
It feels like confirmation bias to select "vertebrate" or "mammal" to show that we are already indeed in a large group.
And indeed you can argue scientists are already applying this logic by looking for signs of simple life, say, or Mars. Or Europa. Because, that's the most likely to be found.
Maybe we should call it the Bacterial Alien Theory. Of course, it wouldn't be very controversial.
But when we restrict our search to sentient life, here is what the statistics suggest [and as you correctly point out, that restriction is what makes this an interesting discussion].
