Most white solar LED post cap lights are sized to fit standard deck and fence posts — typically 4×4, 5×5, or 6×6 posts. The most common size is for a 4×4 wooden post (actual 3.5" × 3.5"), but many models include adapters for 5×5 (4.5" × 4.5") and 6×6 (5.5" × 5.5").