42. I wear size 9 in sneakers (converse chuck taylors and JP's for reference) and I my 42 in CP's are a bit large. I can probably fit into a 41.
Keep in mind that they are not the best for wide feet. I also have wide feet and when I wear it, the widest part of my feet (right below small toe) pushes the shoe and shape by a tiny bit. Most people won't notice it though, it's not a big difference
For reference:
CT: 9
JP: 9
Wolverine 1000 mile: 8
Clarks desert boot: 8
Nike Lunarglide 3: 9
Common Projects Achilles Low Top Sneakers sizing & fit information