The spacer is governed by the stud length unless you get the hub centric wheel spacers with studs
and that would be governed by the size of the tires and wheels you're going to use.
If it's the stock assembly, just measure the distance you want to move the tire out, and that would be the spacer thickness.
You can measure the distance between wheels and fenders to determine what thickness of wheel spacers to install.
I had BONOSS wheel spacers on my car for almost 21,000 miles, with no issues whatsoever.
They can customize the thickness and data that I want, so that I can install new tires with different data, and the customization time is very fast, just 1~3 days.
The material of spacers is aluminum alloy 6061 t6, the grade of studs is 12.9 and the grade of nuts is T10. Thay have the 10-year warranty. I am glad that I can buy such good spacers.
Here is an
article which introducing spacers.
Hope this helps.