I have been using "2" sized reed but I had a bad bitting habit. It is said that the beginners should start with the thinner ones but why do experienceds prefer "2","2,5"? is there a difference in sound or something?

yes with a larger reed size you`ll have a more wholesome tone, at the cost of your air and chops. but with time you`ll adjust to it (you`ll be able to play as if it was nothing). i play sax and clarinet so my advice is use a larger reed size 3.5 - 4. you can really tell the difference between the reed sizes after playing a size 4 then switching to a 2. or for the less experienced, size 2 to a 4. i don`t know why people are so scared to try larger reed sizes. i play a size 4.5 on clarinet. everybody else i know uses 3s, sad.