As a contact lenses wearer, I can tell you that is alright.
In generally, the diameters of contacts are 13.8mm or 14mm. Most brands offer the two sizes, but not every model has both sizes. The diameters are designed according to the material, thickness and curvature of the lenses. Comfortable and safety are crucial.
As for the size of lens, choose in terms of your own situation. Bigger eyes for longer diameter of lenses, smaller eyes for shorter ones. Only 0.2mm difference, do not let the contact lens smaller than your cornea.
No, it is not OK if you wear it. The contact lenses are sticked at the surface of the eyeballs which help you see the things clearly. However, the water content of contact lenses makes your eyes be able to breath the oxygen normally. If the contact lenses are bigger than your cornea. The water uptake ability must be strong which will make your eyes feel dry. Thus, you'd better change one.