recently started playing the tenor sax, and I am not sure which reed should I use. I started with the #2 size, but I can't get the correct air blow when playing notes below A. It's like it jumps higher. It's hard to make it smooth.
It is important to match mouthpiece tip opening with reed hardness so in addition to what Kev asks above it would be good to hear what brand of reed you use.
Also it is best to completely soak a reed in water (or a water/alcohol mixture) for a minute or so in order that it is fully moist and has flexibility. A dry reed may be too stiff and too dry and be harder to play with a tight embouchure. The lower the note the harder your embouchure may be having to work and the greater the likelihood of octave jumps - like squeezing the tip of a hosepipe.
So, soak reeds well before playing and give the tip a gentle massage from back to front. If that does not help then get a softer reed #1 1/2 . Find out what tip opening you are using, as you may need a smaller one - http://www.jodyjazz.com/facings.tenorsax.html
It is usually adviseable to start on a tip opening below 0.090".
What? TomMap not recommended the Bari Esprit yet? Ok I will then.
The mouthpiece is very important in generating a good sound. The ones that usually come with student horns are poor to say the least. The Bari Esprit is a good cheap mouthpiece that is easy to play and has a reasonable tone for anyone just starting out.