Make sure the surfboard bag fits your stick.
The length should be obvious, and the size is also important.
If the surf bag is too small, the surfboard will be jammed into the bag and the zipper may damage the slide rails.
Sacks are marked with length. Such as 6'0, 7'6, 9'6, etc. (Remember to also check the width.)
Most bags are marked beyond 2 inches. So a 6'0 surf bag can usually hold a 6'2 bag, but I wouldn't do that unless it was socks.
I prefer more space than less, especially when traveling with a surfboard bag. It's cool to make a board bag a few inches larger than your board, but don't get too big because you don't want it to move too much inside the bag. But if the bag has internal straps, everything is fine.