The effectiveness of size and color of a security sign depends on various factors like lighting, viewing distance, and mounting height, among others. Like, properties with dim lights would require bright, hard-to-miss signs in yellow/black, or red/white. Parking lots with ample lighting can go with any combination like blue/white, green/white, yellow/white, and more. If the sign is an entrance sign, you can opt for a bigger 24” x 30” size. However, for individual parking stalls, a 12” x 18” sign would be sufficient.