How do you know if shoes are too big or too small?
If you cannot even put on your shoes, or they cause you throbbing pain, that is a clear sign that they may be too small for you. If while wearing and climbing in the shoes you cannot actually benefit from them and do things such as toe hooks because of the pain, then that is also a sign that the shoes may just be too small.
On the other hand, if you feel your feet slip and move around inside the shoes while climbing, then that is a sign that they may be too big. Your feet should not really be moving around inside your shoes while climbing, you should feel the support of the tight and snug fit of the shoes. If you do not feel this, they may be too big. If you have a lot of dead space in your shoes, meaning, air pockets between your feet and the shoes, this may also mean that the shoes are too big, or not the right fit for you.
There is a third option, which is that the shoes are just not the right fit for you. Climbing shoes come in all shapes and sizes. Some shoes have a wider toe box, and some have a more narrow heel area. It can be that you will find shoes that fit the size of your heel perfectly, but, the toe box is either too big or too small. This situation is much less about the actual size of the shoe, and more about the shoe not matching your feet.