How to get the max width for a CIF window

Hi all

I'm building a CIF widget in CRM (Customer Service Hub, to be specific). I'm having a problem where the wider the CIF panel, the better, but if I make it a standard width, like 1000, and someone on a smaller resolution screen uses it, the CIF panel won't open at all.

I'm struggling to find any documentation on how to get the max available width for the CIF window - can anyone point me in the right direction, please?

Thanks
James