There may be a simpler way to do this, but...
1. Open the Inspector Clip tab and for Time Format, choose Samples.
2. While holding the Alt key, click and drag across the selection you want to measure. This creates a clip out of the selection.
3. Click on the Clip handle to select it, then read the length in Samples in the Clip Inspector's Length field.
4. Choose Undo to undo the split.