To give you a starting point, the average square accent tile is between 4 and 6" whereas listello tiles are usually 4"x12", but there are plenty of smaller and larger options.
To find the right size for you, consider your accent tiles in relation to the rest of the room or, more specifically, the area that you're planning on decorating:
Smaller tiles can help you draw the eye onto their pattern and its repetition, and are particularly handy for Mediterranean-style designs;
Larger tiles will result in fewer grout lines, making your room look bigger.