According to a calculator on the web 10 gallons of water will expand a maximum of .445 gallons when hot.
As far as your design is concerned I suggest an expansion tank/vent at the highest point. That's what I did. I also isolated the engine coolant loop from the heating loop that goes to the house. No problems with excess pressure on the engine. Less possibility of leaks that could cause the engine to overheat. I tapped into the the loop that goes to the house to dump excess heat in summer or put some heat in the garage in winter. I thought about it long and carefully and asked many questions before putting it together and spending the $. There is no automation so I do have to watch things a bit. My system has been working perfectly for a year now. I have so much hot water in the summer I waste it on things I never would have before. In the winter, after a couple hours to heat things up, I can keep a large cast iron rad in the living room hot and have enough domestic hot water to take a shower, do the dishes or a load of laundry.
Plumbing diagram one. Outside the house.Â
http://www.genedevera.com/listeroid/cogen/diagram1.jpgPlumbing diagram two. Inside the house. Â
http://www.genedevera.com/listeroid/cogen/diagram2.jpgOther pics at the bottom of my webpage  http://www.genedevera.com/listeroid/
Thanks, Geno