The 'ideal' size would probably be a 56 cm bike but it will be easy to adjust the setup of the 58 cm bike to suit, as described by several of us in posts above.
I normally ride 58 cm bikes with 11 cm stems, and the saddles fairly high and clamped mid-rails. I have been equally comfortable on a 60 cm bike with the saddle nearer the frame and pushed forward slightly, and using an 8 or 9 cm stem. I have also been comfortable on a 56 cm bike with the saddle set high and pushed back, and using a flipped 12 cm stem.
Within reason (and 2 cm IS within reason) what matters is the saddle position relative to the bottom bracket and the bar position relative to the saddle. You can achieve those positions on bikes of different sizes.
Chris - you WILL be able to make the bike fit you. If that bike shop won't/can't help you sort this out, find a better shop or see if you have a helpful CycleChat member near you who knows what they are doing.
Once it is sorted, I'm sure you will have some great rides on that bike!