Server - how do I get client viewport size?

How do I get client viewport size on sever ?

I have some tracing functions, which depend on viewport size to calculate correct trace (as the crosshair is offseted from center of the screen).

What I’d like to have is to get client viewport size, and store it somewhere on server, and then use it for all calculations. If client change resolution, it will be updated on server again.

All tracing is happening completely on server. Client is completely dumb about it and the only thing it gets is end result.

Unless there is better way to do it ?