Trying to pick out new baseboards and door casings in my remodel.

I have 8ft ceiling and was thinking around 5-6" tall for the baseboards, then choosing the proper size casing to match.

Few questions.

If i did 5 1/4" baseboards what would be the correct casing width? I dont think 5 1/4 would work in my place. Would 4" casing be wide enough?

I thought the casing and the baseboards should be the same thickness so that it flows from baseboards to casing. My contractor said that casings are thicker. Does this sound right? All of the pics on Houzz I am using for ideas look like both are same thickness

examples pic

https://www.houzz.com/photos/arlington-traditional-bathroom-boston-phvw-vp~3873201