Guard columns are recommended to protect analytical/preparative columns from contamination from injected particles, worn pump seals/injector rotor seals, or debris from unfiltered mobile phase.
Filtration should be done through 0.22 um to 0.45 um to remove particles and also help degas the mobile phase.
Solid phase extraction or liquid-liquid extraction can also help produce cleaner samples for direct injection.
Not using a guard column will directly contaminate the analytical or preparative column, thus shortening its practical life.
Whenever possible, care should be taken to use the same material in the guard column as in the analytical/preparative column, especially when performing method development.
Typical analytical guard columns are 1 or 2 cm long, 2.0 (2.1) or 4.0 (4.6) mm depending on column size, and 1 cm long for 10 mm, 21.2 mm, and 30 mm ID columns.
Whether you choose a 1 cm or 2 cm long guard column depends on how harsh your mobile phase is and how messy your sample is. Ideally, to preserve your chromatography and avoid adding stress to your system, you should use the shortest guard column available and use the same internal diameter as your column if possible. Otherwise, you should choose the closest possible guard column with the smallest internal diameter.