You would be able to find answer as below.

URL :http://www.highveld.com/pcr/real-time-pcr-design.html

Amplicon size: The shorter the better. With taqmqn probes, 50-150 bp.

Distance forward primer to probe: Maximum 50 bp. 

Dear Tsuguto Fujimoto,

Thank you for your answer. 

Dear  Nathanael

Adding to previous:

Overlaps between primer and probe can be 3-4 base pairs. Longer overlap will lead to

primer-dimers. Also you can use modified TaqMan(Minor groove binder for example)

and primers in relatevely GC rich region of DNA. In summary it will gives as short

amplicon as possible.