This was made to look perfectly in-game with any resolution.

cl_crosshairsize "3";
cl_crosshairthickness "1";
cl_crosshairusealpha "1";
cl_crosshairalpha "255";
cl_crosshairgap "-2";
cl_crosshair_drawoutline "0";
cl_crosshair_outlinethickness "0";

cl_crosshairdot "0";
cl_crosshairstyle "4";
cl_crosshaircolor "5";
cl_crosshaircolor_r "255";
cl_crosshaircolor_g "0";
cl_crosshaircolor_b "0";