What does it mean for a planet to be just 300 kms across? The recently discovered Planet Goblin is just 300 kms across. While I perfectly understand what it literally means, I'm having trouble wrapping this in my head. Does it mean I can imagine the planet as (approximately) a sphere for 300 kms diameter? That gives a radius of 150 kms, and thus a circumference of 942 kms. 

A circumference of 942 kms means that the longest distance between any two points on the surface of the planet is just 471 kms -- That's just over 1.5 times the distance between London to Paris or 100 kms more than the distance between New York and Washington DC or about half the distance between St. Petersburg and Moscow. 

Is the planet that small, really? Or am I missing something?
The reports that Goblin is a planet are incorrect. It is a dwarf planet, at best. Dwarf planet and planet are mutually exclusive terms. Planets are objects whose masses are so large that they have collided with, ejected, or captured the vast majority of the smaller stuff that share the same orbit as the planet.

Moreover, Goblin might well not even be a dwarf planet. The distinguishing characteristics of a dwarf planet is an object that is large enough to have pulled itself into a more or less spherical shape via self gravitation but is not anywhere near large enough to have the potential to clear the neighborhood of its orbit about the Sun.

The threshold that distinguishes dwarf planets from small solar system bodies is the potato radius. Objects somewhat smaller than the potato radius look like lumpy potatoes, while objects larger than that have sufficient mass to more or less pull themselves into hydrostatic equilibrium. The potato radius depends a bit on composition, and probably also depends on distance from the Sun. (Water ice is quite rocky at trans-Neptunian distances from the Sun.) The potato radius for rocky objects is about 300 km (diameter = 600 km), while for icy objects it's about 200 km (diameter = 400 km).

With an estimated diameter of 300 km, Goblin is probably too small to qualify as a dwarf planet, let alone as a planet.