Can I use the other surgery cutting boards and how big should I make them?
Yes! Douglas fir makes fantastic end-grain cutting boards with dramatic swirls. The thickness is really a personal preference depending on the finished size of the board. I would suggest making a 20x12 board 2" thick. I wouldn't suggest ever making an end-grain cutting board less than 1" thick.
I do have a few suggestions if you're talking about using construction-grade lumber from the big-box store. First, get 2x10s or 2x12s instead of 2x4s. These have far fewer defects and will make a nicer looking end piece. Second, dry the wood out in your oven @215 for 2-3 hours before you really start working with it. This will prevent the wood from possibly splitting at some future point.