I heard that egg size should be 18mm or more in order to conceive. is it impossible to conceive with egg size less than 18mm?

Follicle size: 18 is just an optimal size, but certainly eggs slightly smaller and ones slightly larger ovulate as well. I was always taught that 16 to 21 mm are a good sized mature follicle.

Size a normal egg: Actually I believe that you are referring to the follicle size measured by ultrasound prior to ovulation, which is around 16-20mm. The human egg is actually much smaller than the follicle which contains the egg.. Nevertheless there are many preovulatory follicles smaller than 18 mm that can result in a pregnancy.