My labia minora is of different sizes; one side is bigger. is it normal?

Yes.: Some asymmetry is quite normal. If aesthetically or functionally "symptomatic", labia minora reduction surgery may be the answer.

Asymmetry is normal: There is no standard as to what labia should look like. It is what you are comfortable with. If you experience pain, discomfort in clothing, with activity or during intercourse or have frequest utis and yeast infections then you may want to look into labiaplasty. Please do not feel that there is a "normal" labia.

Very likely normal..: You are very likely perfectly normal. There is natural variation in all paired body parts...Breasts, fingers, etc. :).