You might be able to find a size range for the beret on the manufacturer's website. Higher end berets usually come in different sizes, so you can find the perfect fit.

What size beret do I need?

Determining your beret size: Measure the widest part of your head with a tape measure. Place the tape measure an index finger above the ear; bring it across the mid forehead, completely circling the head. ... If your measurement falls between sizes, choose the next largest size.

How are Berets supposed to fit?

Align your beret properly.

This typically means to wear your beret with the brim straight across your forehead. The United States Army requires the edge to sit one inch above the eyebrows. If your organization requires a "flash" insignia, it should be aligned over your left eye.