There are a number of legal console commands you can use to adjust your crosshair. I have put the commands, and then in brackets the values you can use (you can use higher/lower values than I have suggested, however you will likely end up with an unusable crosshair).
cl_crosshairsize x (0 – 10)
This command simply changes the lengths/size of the vertical and horizontal lines of the cross hair, use 0 to turn them off.
cl_crosshairdot x (0/1)
This command if set to 1 uses a dot at the center of the crosshair, if set to 0 then it removes the dot.
cl_crosshairthickness x (0 – 5)
This command increases the thickness of the dot and the lines of the crosshair.
cl_crosshairalpha x (0 – 250)
This command sets the transparency of the crosshair.
cl_crosshaircolor x (0/1/2/3/4/5/6)
Sets the colour of the crosshair to one of the set colours.
cl_crosshairgap x (-15 – 10)
Sets the gap inbetween the lines of the crosshair
And now a couple other examples:
OK, so go and have a play and come up with a crosshair that you feel comfortable using.