Many thanks for the competent, quick answer!
In the Lenovo V520s SFF, there is an installation space of approx. 180 mm in length and 80 mm in width, as well as the matching socket strip on the mainboard.
So I ordered the graphics card they suggested and am installing it today. If there are any problems with the installation, I will report them here. Thanks and greetings Peter

I would recommend this one: Gigabyte GeForce GTX 1050Ti OC (4GB, mid-range)

This will give you a bit more performance. However, I would briefly measure the length of 16.7cm in the case. The low profile bracket for the SFF PCs is included in the package.