This is a difficult question since so much depends on your specific target sequence and application. A good set of rules for the design of molecular beacons can be found on www.molecular-beacons.org. In general, the most successful compounds are 30-40 nucleotides in length, with the stem comprising the last 6 bases on either terminus. Another general rule is that the melting temperature of the stem should be about 10°C lower than that of the target/probe section of the molecular beacon. This is where the trial and error comes in as the design tools are only predictive of nature.