I have one front tooth that is small than other surrounding teeth. i want to know that my tooth size can be lengthen by crown lengthening? any risks?

Small or covered?: There is a big difference between a tooth that is smaller in total size as compared to the adjacent teeth, and\or the corresponding tooth on the other side vs. A tooth that is not fully erupted or covered by excess gum tissue. This doesn't sound like a case where crown lengthening is indicated. When you perform a crown lengthening procedure, you remove bone & bone is precious! see your dentist.

Cosmetic lengthening: With a proper diagnosis, and a competent surgeon, there should be no problem with such a procedure. Just make sure that the dentist properly determines whether it is only a problem involving gum, or involving both gum and the underlying bone, as the surgical approach for each of these situations is quite different.