Too small contact lenses will probably irritate your cornea and cause a fair bit of discomfort. On the other hand contacts that are too big will also irritate the surface of your eyes from moving around too much and not applying pressure evenly across the surface of the eye. You may also encounter some problems while inserting a poorly fitting lens - its not going to want to sit on your eye properly. We highly recommended that you get your eyes tested and perform a lens fitting session before buying a pair of contact lenses as wearing a poorly fitted pair may lead to eye irritation and even eye injuries. Your local optician will be able to do this for you and again we highly recommend that you do this before buying contact lenses for the first time, even if you don't need a prescription to buy contact lenses in your region.