I have a tooth decay. During the treatment the dentist sayed that I have very long root and more than usuall. What does she mean?!

Genetics. Root length varies among individuals. Body height and weight do not determine root size. One may be petite yet have long tooth roots. I usually see this trait as favorable. The longer the root the better supported the tooth or teeth. Also, more amenable to treatment if it needs a crown extension for instance.
Normal variation. Some people have long roots, some short. Your dentist was commenting on a normal variation. Ask her to show you on your x-ray next time you see her.
Longer than average. I would guess that this comment was mentioned just in passing. All that it means is that your dentist happened to notice that compared to most comparable teeth in other patients, the roots of your tooth\teeth were longer. It's just an individual variation and better to have longer rather than shorter roots for tooth strength. Short roots can be a problem.
Dental anatomy. There is an average length of teeth for most people--but just like noses--some teeth can be shaped differently than average. Despite the length of your tooth, the endodontic therapy (root canal) can still be performed usually.
Depends on treatment. Your doctor may have just mentioned it in passing but it may have greater implications later on. You may require a specialist to either perform a root canal or remove it at some point in the future as a long root can be very difficult to treat. It is also beneficial in that it requires much more bone loss to be considered unsalvageable. It all depends on how well you take care of your teeth!