You can customise your crosshairs using the cvars g_crosshairCustom, g_crosshairColor, g_crosshairSize and g_crosshairCustomFile.

g_crosshairCustom specifies if you want to use custom crosshairs
g_crosshairColor is used to set the crosshair colour ( R G B Alpha)
g_crosshairCustomFile stores the custom crosshair's filename
g_crosshairSize sets the crosshair size which must be 16,24,32,40,48

Example:

seta g_crosshairCustom "1"
seta g_crosshairCustomFile "gfx/guis/crosshairs/mycustomcrosshairs.tga"
seta g_crosshairColor "0 1 0 1"
seta g_crosshairSize "48"