I am guessing your Video Resolution is set low, (eg 800 x 600)
I am no techie like the guys we have here So maybe post this in the TECHIE section if you don't get your answer in the next day or so.
but anyway to your question, your crosshair can be made smaller just by going to options and i believe its one of the settings, and it will pick crosshair settings, click it and you can make it smaller or bigger choosing on which 1 u want. tbh mine is about 2 mm big on the screen XD. ( existance not for you to comment XD nor anyone else XD)
but thats how i changed mine. i hope that gets you goin tin the correct direction
blue i have the medal of honor HUD for DOD:S its pretty cool it allows you at the begining of the match to pick the crosshair you like and the size... it can get pretty small and i use the standard + crosshair... theres a command you can type into console to resize it but it also makes it less visible sorry but i forget what the command it