These post covers will cover a 6x6 (5.5" x 5.5" ) post and can have a cut out created to allow a mailbox to come through
This product is 8 inches thick.
Yes they absolutely can be used to cover basement poles!
Yes these are made to fit nominal 6x6 posts (5.5" x 5.5").
NextStone makes a Post Cover Converter Kit where they are square rings that fit a nominal 4x4 building the post out to a nominal 6x6 for the Post Covers to fit. Here is the link for them: https://www.homedepot.com/p/NextStone-Slatestone-Post-Cover-Converter-Rings-4-Pack-SLS-PC-CK/300243312?keyword=SLS-PC-CK&semanticToken=222400+++%3E++++st%3A%7Bsls-pc-ck%7D%3Ast++cn%3A%7B0%3A0%7D++sls-pc-ck+%7Brest%7D++dln%3A%7B567166%7D+qu%3A%7Bsls-pc-ck%7D
Yes, each piece covers 2 sides.