Determining the size of birds can be done through observation, reference guides, or online resources. Here are a few methods to help identify the size of birds:

1. Observation: Spend time observing the birds you want to exclude from the area. Pay attention to their general size in comparison to common objects or other birds that you are familiar with. Note their body length, wingspan, and overall proportions.

2. Field Guides: Utilize bird field guides that provide information on various bird species, including their physical characteristics and sizes. These guides typically include illustrations or photographs, along with measurements or size comparisons to assist in identification.

3. Online Resources: Numerous online resources offer comprehensive information about different bird species. Websites, databases, and birding often include photos, descriptions, and measurements that can help you determine the size of specific birds.

4. Local Birding Groups or Experts: Reach out to local birding groups, ornithologists, or bird experts in your area. They can provide insights into the common bird species and sizes, based on their knowledge and experience.

It's important to note that bird sizes can vary within species, and there may be subtle differences between males and females. Therefore, it's helpful to gather information from multiple sources to get a comprehensive understanding of the size range for the specific birds you are concerned about.

The choice of mesh size for anti-bird netting depends on the size of the birds that you want to exclude from the protected area. Different bird species vary in size, so selecting the appropriate mesh size is crucial for ensuring that the netting effectively prevents access for the target bird species.

Here are some general guidelines for mesh sizes based on the size of the birds:

1. For small birds such as sparrows or finches, a mesh size of around 0.5 inches (12-13 mm) is recommended. This prevents smaller birds from squeezing through the netting.

2. For medium-sized birds like pigeons or starlings, a mesh size of around 0.75 inches (18-19 mm) is suitable. This prevents these birds from getting tangled in the netting or finding openings to enter.

3. For larger birds such as seagulls or crows, a mesh size of around 1-2 inches (25-50 mm) is typically used. This provides a sufficient barrier to keep out larger birds.

It's important to note that these are general guidelines, and the specific mesh size may vary depending on the bird species in your area. You may want to consult with bird control experts or suppliers of anti-bird netting for more specific recommendations tailored to your situation.

If you have any further questions, feel free to ask!