I need a lot more info for sure and I really dislike lighting that lights the ceiling and not the stairwell or the entry . I also have no idea about the rest of the lighting or your style so please more info and the reason you like this piece and please think long term as to the type of bulb you want in any light you choose since changing the bulbs will be a job to never want to do so all LEDs for sure

Hello! You definitely need a longer chandelier for stairwell. Something like this. Please, consider these variants

Staircase Lighting Ideas · More Info

Staircase Lighting Ideas · More Info

Staircase Lighting Ideas · More Info

I like those but we need some info from the OP

im not sure how wide you think it needs to be, the eye will read the entry, where the railing to the wall is, and the doorway in front of it, I wouldnt go much larger.

How low should the chandelier be in a 20 foot height entryway

The fixture is 42" tall !!! It should fit just fine. I would change it so that the bottom is just below the underside of the landing in your photo.

This fixture is available at HD. 2 slightly different styles but the price is much lower. I like the version with the added brass details for your leans traditional home..

https://www.homedepot.com/p/Magic-Home-12-Light-Black-Antique-Gold-Lantern-Tiered-Hanging-Ceiling-Chandelier-MH-Y-020213/317499656

https://www.walmart.com/ip/Magic-Home-Industrial-Traditional-Vantage-12-Light-Lantern-Rectangle-3-Tier-Chandelier-Ceiling-Mount-E12-Adjustable-Hanging-Height-Black-Antique-Bras/337018309?wmlspartner=wlpa&selectedSellerId=101061678

Amazon for the same light fixture

7ft from floor at the lowest, in your case you may want to look at it per how it will fall to the center of the stairwell area.