XRJointPose.radius

navigator.xr.requestSession({optionalFeatures: ["hand-tracking"]}).then(
  // ...
);

function renderFrame(session, frame) {
   // ...

   for (inputSource of session.inputSources) {
      if (inputSource.hand) {
        let indexFingerTipJoint = inputSource.hand.get("index-finger-tip");
        let radius = frame.getJointPose(indexFingerTipJoint, referenceSpace).radius;
      }
   }
}