The first thin you will want to consider is where you are getting the tattoo. What are the specifics of the size that you want. Sure small works but the tattoo artist designing the tattoo will need to know more specifics then just small. It can be helpful to get out a ruler and actually measure the size of tattoo design that is desired. Another consideration with small tattoos and there actual size is the location of the tattoo. For example a small tattoo design for the whole back will be too big for a foot.
It is also important when considering a small tattoo design to think about the amount of space available. It is important to think about the space of the overall design and the space between the elements of the design also. When things are small they can quickly become too much and overcrowded. A cleaner and simpler look typically works best with small tattoos as it will leave enough space for everything to be seen clearly.
Another thing to consider with any tattoo is the long term. Sure you might want a small, delicate tattoo with a very intricate look and lots of detail. However consider the amount of time you will have the tattoo. Tattoos are pretty much forever so the original tattoo work can start to spread through the skin. The problem is the lines start to blend together and one can quickly loose that delicate complexity of the tattoo over time. So it is important to think about this when planning out the design.
Another consideration when looking for small tattoo designs is the amount of detail. The smaller the tattoo is the less detail that can be put into it. If you want a very elegant tattoo with a lot of intricate designs then going small is pretty much impossible.