More generally, one can interpret the statement in both of two ways:
1) Mankind is the literal measure of all things; the physical characteristics of a thing can be expressed in terms of a human being. In other words, an object can be described using a human being as a yardstick or standard mass or standard volume, etc.
For instance: “The Dragoth tended to be half-again taller than the average human male, and some even doubled the proportion.”
Or: “For all their size, most Libo tipped the scale around a tenth of the mass of an average human being, having evolved in an extremely low-gravity environment.”
2) Mankind is the abstract measure of all things; the value (or worth) of a thing is expressed in terms of its usefulness to human beings, practical or otherwise. The less human beings need (or believe they need) an object or resource, the less they value it.
For instance: “Another diamond. Fayci tossed it into the box with hundreds of others. With the carbon planet Mantium just next door, the things were as common as dirt in this star system. She kept digging.”
Or: “An ammonia re-breather. Great. A welcome find for an Ammoniak, maybe, but it wasn’t even valuable as scrap to Griegson at the moment. She took another quick breath, silently cursed the audible low-ox warning from her suit system, and smashed open another cargo container.”
In the topics and discussions on this blog, I’ll be using both of these interpretations of what I like to call the Protagorean Principle (emphasis on the “e” syllable, \prō-ˌta-gə-ˈrē-ən\).
It’s not that I don’t like aliens. The environment and ecosystem of a Pandora™ or a Dune™ are fascinating, and my hat’s off to those with the time, knowledge—and, above all, patience—to develop them.
Human-Habitable and Human-Inhabitable
However, these environments are interesting chiefly in the contrast they create with human-habitable environments, such as we find on Earth, and (potentially) human-inhabitable environments, such as we find on Mars.
So, for the most part in this blog, I will be focusing on the kinds of planets that are either immediately habitable by humans (thus, they are human-habitable), or are potentially inhabitable by humans (thus, they are human-inhabitable), as well as the kinds of galaxies and star systems in which such planets are likely to be found. Further, I will tend to refer to such bodies as worlds to distinguish them from “mere” planets.
In 2006, the International Astronomical Union (IAU) famously demoted Pluto from planet status in a decision which “…has made a lot of people very angry and been widely regarded as a bad move.” 
For a refresher, the new definition of “planet” runs like this:
1. A planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.
2. A dwarf planet is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
3. All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies. 
Tim DeBenedictis in an article  at Space.com, makes a simple and compelling argument that the dividing line should simply be set at objects of greater than 1000 km radius which independently orbit the Sun. At 1000 km radius, he says, the materials of which a body are formed, be they rock, ice, or metal, will start to deform under their own gravity and the body will become spherical(ish).
His definition  is rather simpler than that of the IAU:
1. A "planet" is a celestial body that (a) is in orbit around the sun, and (b) has a maximum surface radius greater than 1,000 km (620 miles).
2. All other objects, except satellites, orbiting the sun shall be referred to collectively as small solar-system bodies.
By his definition, all of the bodies shown below qualify as full-fledged (terrestrial) planets:
1. A "planet" is a celestial body that (a) is in orbit around a central star or stars, and (b) has a maximum surface radius greater than 1,000 km (620 miles).
2. All other objects, except satellites, orbiting a star or stars shall be referred to collectively as small stellar-system bodies.
…and I’m adding a third point:
3. A “world” is any of the above which possesses a solid or liquid surface which, itself, is either a) human-habitable, and thus possesses and maintains a viable, active biosphere which can directly support human life, or is b) human-inhabitable and thus can be easily inhabited by humans through the use of artificial habitats.
Yeah, I know how wide-open I’ve left myself, there. What does “easily” mean? For the sake of simplicity, I mean a Mars-like or Moon-like environment, as opposed to a Venus-like or Europa-like environment.
Speaking of Europa, I guess I could say that I’m using “world” here in the same sense as Dave Bowman used it in his last message to Earth at the end of 2010: “All these worlds are yours except Europa. Attempt no landing there.” 
So, when I refer to “habitable stars” and “habitable galaxies”, I’m referring to those around or within which worlds might reasonably be found.
I hope you’ll consent to come along on this voyage, and that you’ll be prepared to be gentle and forgiving if I stumble or outright fall flat on occasion. As I've said elsewhere, I'm no expert.
2. "IAU." International Astronomical Union. Accessed January 07, 2017. <http://www.iau.org/public_press/news/release/iau0603/>
3. "Why Pluto Is a Planet, and Eris Is Too." Space.com. Accessed January 07, 2017. <http://www.space.com/29571-why-pluto-is-a-planet-and-eris-is-too.html>.
5. 2010. Produced by Peter Hyams. Directed by Peter Hyams. By Peter Hyams.
6. Zeigler, Jon F., James L. Cambias, and Wil Upchurch. GURPS space. Austin, TX: Steve Jackson Games, 2006.; <https://www.amazon.com/GURPS-Space-Fourth-Generic-Universal/dp/1556342454>