The REVIAN cap is designed to fit head sizes 54cm (21.25") to 64cm (25.2"). This corresponds to fitted adult hat size of 6 3/4 to 8 (Small to XXL)
If you find that the cap fits loosely on your head you experience difficulty connecting the cap to the REVIAN app. The reason is that the sensor located in the front inside of the cap must make contact with your forehead to "wake up" the cap and allow for it to broadcast via Bluetooth.
The cap is designed such that the headliners inside the cap are removable. The rear headliner should be replaced with a thicker headliner supplied in the accessory kit shipped in a box under the charging base. This accessory kit is the same that contains the AC adapter and charging cable. The thickest headliner is approximately 30mm thick and should allow the cap to rest against your forehead. If you have long hair you will need to ensure that your hair is pushed to the side to allow the cap to make contact with your skin. The cap does not necessarily need to be or feel tight as long as the sensor detects that the cap is on your head.
Once connected to the REVIAN app you will be notified via an on-screen alert if the sensor isn't making contact. An image and message instructing you to place the cap on your head will be displayed.