That's called the Pilot crosshair. or at least that's half of it. It also comes with a vertical line that you enable seperately. A lot of the longer running HUDs have them installed in hudlayout.res like HUD crosshairs
I've never really messed around with large res VTFs. What's the limitation? Only limitation I know of is the width and length having to be a power of 2 and I've seen large resolution crosshairs before so it's probably very doable
edit: made a quick resize of a crosshair to 2048x2048. seems to work just fine https://imgur.com/a/5f70JeM