According to the Cannabis Grow Bible you should wait until about the 3rd or 4th week of vegging to take a clone.
"The best place to take a cutting for cloning is above a node that has at least two nodes above it. The smallest cuttings on average are three inches in length." ~page 166.
As for sexing, this is also from the Grow Bible
"'Force-flowering' is probably the best early-sexing method. To force-flower a cannabis plant, simply take a cutting and place it in a cup of water or a cloning medium, such as rockwool. Expose the cutting to 12 hours of light followed by 12 hours of total darkness. The cutting should flower and display it's sex -- however the plant must be mature enought to present its sex. An immature plant will not show sex because initial calyx development is not photoperiod-related. Plants normally mature around the forth week of vegative growth because sex is not genetically determined until the third week of growth." ~page 102