What size wire should I use?

I'm having someone install some light and sockets in my shed. The lights are 8' fluorescent fixtures with 4 bulbs each, 3 will be put in. There will also be 3 outlets installed around the shed. The main panel is about 45' from where he's going to mount the sub panel. I'm going to buy the wire that will feed the sub panel but the guy isn't answering his phone, how big should it be?