The Solar can use an action to release its sword, causing it to hover within 5 feet of it:
Flying Sword. The solar releases its greatsword to hover magically in an unoccupied space within 5 feet of it. If the solar can see the sword, the solar can mentally command it as a bonus action to fly up to 50 feet and either make one attack against a target or return to the solar's hands. If the hovering sword is targeted by any effect, the solar is considered to be holding it. The hovering sword falls if the solar dies.
I could not find any size / space requirement for the sword in this description, and if it takes up actual space (like a creature would, e.g. being large and therefore occupying a 10ft. by 10ft. space). Are there rules / information that can answer these questions?
Best Answer
It's not specified, but there are some guidelines.
The Solar's ability reveals that the sword must indeed occupy an empty space (just like a creature would):
From this answer, a standard greatsword counts as a medium-sized object. The Solar is a Large creature, so your DM could rule its greatsword was also large, but that's something for you to discuss at your table.
Bottom line, it's not clear by RaW, but you could draw some conclusions from the way the ability is worded and how Animate Objects works. Personally, when playing on a grid, I'd have it occupy a 5-by-5 ft square, just like a medium creature. However, the sword won't trigger or make Opportunity Attacks, since it's still just an object.
Now, simply occupying a space doesn't mean it's impassable. Your DM can describe how the space is occupied, and how the characters can handle it.