I'm definitely not an expert but I've always heard that if in doubt, it is fairly safe to prune 1/3 of height (or length of branches). I'm reporting this as a general guideline for pruning.

I don't know much about Rosemary except that it has a deep root once established so may be hard to kill. You can probably have fun with shaping it however you'd like.

I know you can trim rosemary into topiaries. :-) I'm pretty sure it'll hold up to whatever you throw at it.

Here is a link that might be useful: Rosemary topiary

Nah, don't worry. Rosemary takes pruning really well. Whack that sucker to 3-4" and it will come back as good as ever!

Omar

I've been growing rosemary in my yard for 13+ years. Sometimes they will start to die out in the center which isn't all that uncommon. I have found when the plant starts to get very old and/or if it does not get enough water is when I see the die back. Keep them watered, and let them dry out, but remember they cannot live in standing water, i.e. they don't like wet roots.

What I do, and did yesterday as a matter of fact, is just remove the dead limbs all the way down and the lower limbs that are on the ground. They always liven up shortly after that. I don't prune them down, because I like them to be large. None of mine (I have around 15 different varieties of them.) have ever exceeded 4' x 4' in the ground. Some stay smaller, and the prostrate makes for a great ground cover.

And I agree with elphaba about not pruning it back more than 1/3. You can, but then you stress the plant. Rosemary are hardy, but not indestructible. And remember that if you are going to prune it, wait until after the flowers have bloomed. One website that I read from a Master Gardener in Arizona states the following:

Rosemary tends to become bare at the center and should only be to be (sic) pruned lightly out at the leafy wood. Limbs pruned back to the bare wood will not grow. Prune in April.

Personally, I would advice against pruning them into topiaries. I know that lots of places do them, but like the little Christmas tree rosemary topiaries, . . .well the ones I've had start to die out shortly after the Christmas holidays. So I took them out and planted them in the ground and let them go back to their natural shape. Keeping them pruned as a topiary is too much work for me. They are doing great now. The extreme pruning on them was putting them into stress. However, I know lots of people like the topiary look.