The thickness columns is a function of their height and loading. Steel columns (lally columns) are typically 3 inches or more in diameter with a wall thickness of about 3/16 of an inch. Standard top and bottom bearing plates are four inches by four inches by 1/4 inch thick. Larger top bearing plates may be required to support wood beams.
Built-up wood columns should be bolted together with 2 bolts every 16 inches, typically, or nailed together with 2 nails every 12 inches. The bottom of wood columns should not directly contact concrete. Round wood columns should be 7 1/4 inch diameter, minimum, typically. Square solid wood columns should be 6 inches by 6 inches minimum.
Hollow masonry columns should be at least 12 by 12 inches or 10 by 16 inches. Round concrete columns should be 9 inch diameter, minimum, and square concrete columns should be 8 inches by 8 inches, minimum.
Multistory houses or large concentrated loads require larger columns. Columns may buckle if they are too slender.
In some cases, a mason's level is helpful to determine whether a column is buckling, although usually it is visible. Reinforcing wood columns to prevent buckling is usually easy, as long as the movement has not been great.