UI doesn't scale based on resolution and HUD is relatively small already in 1920 x 1080 (let alone in 4K), and crosshair in particular is difficult to see.
Having non-transparent HUD would help playing at high resolutions. How can I disable HUD transparency?
Best Answer
I've tried changing some parameters in the console, but none of the known commands improved the HUD visibility. With hud_draw 1 the HUD is in all probability at maximum opacity.
This leads me to to believe the transparency of the entire HUD - including the in-game text messages - is hard-coded. To change it, you would have to recompile the applicable scripts.
What you can do for increased effectiveness, is change the sprites that are used.
These can be found in the 'half-life/valve/sprites' folder, and the file with the crosshairs is appropriately named crosshairs.spr. You can use a number of small programs to open these files, and convert them to workable file formats. I've used Sprite Explorer.
Naturally, there are edited versions available already, and these ones, made by w00tguy123 on the SvenCoop forum, might help you out.