Just had a quick look at how to do this (draw bounding box for non-grid-aligned circle):

1. Make sure you have object-snapping enabled
2. Snap to the circle's highlighted center snap point
3. Move straight up to the top of the circle until the 'quad' snap point appears
4. Create a horizontal construction line from that point
5. Move along the construction line until 'Square' appears at the circle's centre
6. Click and you're done


Edit - Pilou posted similar reply whilst I was typing mine