The size of a rabbit nesting box depends on the breed and size of the rabbits. However, a general guideline for the dimensions of a rabbit nesting box is approximately 12 to 14 inches (30 to 35 cm) in width, 18 to 20 inches (45 to 50 cm) in length, and 10 to 12 inches (25 to 30 cm) in height. These dimensions provide enough space for the doe (mother rabbit) to comfortably enter the box and nurse her kits (baby rabbits) while providing a secure and cozy environment for the kits.

It's important to ensure that the nesting box is constructed with solid materials that provide insulation and protection against drafts. The box should have a low entrance that allows easy access for the doe but prevents the kits from escaping too early. Additionally, providing bedding material such as straw or hay inside the nesting box creates a soft and warm surface for the kits.

Keep in mind that larger or smaller breeds may require adjustments to the dimensions of the nesting box to accommodate their specific needs. It's always a good idea to research breed-specific recommendations or consult with experienced rabbit breeders for more precise guidelines.