Suggest and test for the visuals. Width: from wall to wall and Length: just account for required space for functionality and movement near built-ins. If too far down though, rug placement will become an issue. If interested (now would be the time), a linear chandelier would really help balance the room and center over the table vs in the centre of room).

I’d center the table and chandelier as a pair rather than trying to center each one independently in the room. With the built-ins and doorway creating different visual boundaries, the relationship between the table and fixture will matter most.

And if you haven’t purchased the chandelier yet, I’d also consider something with a softer, more sculptural shape here. A moon-inspired pendant could look beautiful over this table and would give the room a more distinctive focal point.

I recently compared several moon-inspired lights, including one pendant chandelier that might give you some inspiration:
https://youtu.be/g-R98L8EdII

center on the window and the left wall (the space where the table will actually sit with chairs comfortably placed around the table)