Hi @jrgordon, and thanks for your query. I am afraid that the Tobii XR SDK does not expose pupil diameter with this API.
To accomplish this, you would need to use the Tobii Pro SDK which you can download @ https://www.tobiipro.com/product-listing/tobii-pro-sdk/
Whilst pupil size is exposed in the .NET, Python Bindings, it may not be available in the Unity API, please check with Tobii Pro Support about this @ https://www.tobiipro.com/contact/support/
Please do let us know if we can provide any further information. Best Wishes.