A better method would be to simply use either medians or minimal values, for tiers that already have calculated medians. For instance, Country level, where we know the average country's surface area, City Block level where standard city blocks are widely available, and Mountain level, where the minimum mountain height is defined as roughly 600 metres and we can estimate volume by getting the smallest mountain/largest hill, which'd be around that height, and scaling it.
It's safe to note, however, that the average mountain would still be much greater than 600 meters. So cases like that are discussable.
Cities and islands are utterly arbitrary due to the definition varying a lot from location to location, so we'd just have to choose a relatively arbitrary, but still reasonable (several kilometers and up) landmark. That's unavoidable, sadly, and not something we can fix.
However, logic by itself can still let us know not to just go by the lowest ever and make reasonable assumptions that the majority of fiction should follow. When we talk about a city, we're thinking of a place where people have to drive around for a while in order to reach their workplace, a place where you can stand on the rooftop of an average building and only see the sea near the horizon, and overall a location big enough to house many city blocks within a single mile, as well as have business as usual.
Your average apartment has 10ft in one story. So if you assume two stories, then you have roughly six meters. So the city we're talking about would be at least eight kilometers even if we assume this building is near the edge.
Paris is considered a small capital compared to most others as per architects, and even then it's 10km. So the average city is certainly on that range or a little below, 8km like I said. Likewise for an island, which in fiction are almost always populated by ranges of mountains. Almost every island in fiction shows mountains in their horizons. The average island in most franchises is definitely higher than the >dozen kilometers mark.