You can prune quite hard back just so long as you remember the following:
Make a slanted cut just above an outward-facing bud eye. Cutting at a slant helps water run off of the wound, which prevents water from collecting. After making this cut, the rose will direct its growth to the closest bud, sending out a new terminal shoot. Choose an outward-facing bud eye to ensure that the new growth is directed away from the center of the plant.
Roses are very hardy.