The manufacturer recommends using LINX with smooth finished lumber; dimensions of 3.5"x3.5" or 5.5"x5.5". The extra space between the lumber and inside edge of bracket is not an issue. This space makes it easier to insert the post/beams. If the inside dimension of the bracket were any tighter then it would be extremely difficult to insert lumber and may require sanding if the wood has expanded from moisture in the environment.