When choosing the right loop, you not only want to think about the style or feeling you're trying to achieve, you should also think about proportion, generally speaking you don't want to have your loop to overwhelm the bead - unless the loop itself is part of your design.
For instance: A small loop works with petite beads and a formal, precise design.
The standard loop (about 4 mm) works when the loop is simply a matter of connection.