What should be average size of erect penis which is considered normal?

About 5 inches. Sizes very variable & genetically predetermined. 4" is small normal, smaller than 4" is usually abnormal for Caucasians, Hispanics & Africans (including Afro-Americans, afro-Caribeans etc., up to 7'' is certainly normal. Some Asians normally have smaller penises. Don't be influenced by mega-penises as seen in porno movies & magazines.Very long penises are frequently not appreciated by sex partners.