First make sure you are zoomed in to a scale that is appropriate for the feature you are labeling. Once labels are converted to annotation, they are locked to the size they were at the point they were generated. Right click on the layer, and select Convert Label to Annotation.
Any labels that will not fit on your map at the chosen scale will be placed in an Overflow Window. Select individual items from the overflow list to zoom to, pan, or place the remaining annotation.