These come in small (1080 res), medium (1200 res), and large (1440 res).
I've worked primarily on what I call the large set to work on 1440 res, but even those icons are smaller than what ASI called medium. The goal was to rework them to to have less white highlighting and to have icons standout without being too obtrusive. Though some icons are made bigger for strategic reasons, such as ACU, nukes, etc.
You can find these in the mod vault, but since un-scouted icons and support commander icons do not load, you need the additional nxt file loaded into c:\programdata\faforever\gamedata folder in order for those to show up.
Whether you download the main mod from here or the vault, it has to enabled in mod manager, same as any other mod.
If there is interest I can work on a huge set for 4k, but I don't have a 4k monitor to test.
I've tried to keep the small set as close to original size, but in many cases they are a bit larger, with more pronounced shading, so some may still prefer the original size for attack units, but you can simply delete the icon_land,bot,ship,sub icons from the custom-strategic-icons folder and keep the structure and or land icons that you prefer.
Many of the icons have slightly different sizes, profiles, to help distinguish them, but its subtle difference.
For example, ACU's have individual icons to help those standout.
Also included are T4 icons from IKIT.
Feel free to modify as you see fit.
Here is alternative download in case the uploaded files don't work.