They are a bit on the tight side so if you normally alternate between two sizes, pick the larger one. If it's the Vertigo or Vertigo Corsa it's adjustable anyway

 

I got Vertebra 2s. Im a UK size 11 in shoe/trainer (sneaker or whatever) and i got a size 11 boot.
They were tight to start with and they are slightly adjustable, but theyve loosened up a bit now and fit like a dream.

 

I was on a website, maybe kneedraggers, but they said Sidis tend to run a bit big. AS and AGV tend to be a little narrower, or so I was told/read. I wear a 13 shoe/sneaker, and I ordered a set of Vert 2 in 48, which Sidi says is a 13-13.5. I paid $212 shipped off Ebay yesterday.

 

I have the vertigo corsas. I ordered the same size I normally where in shoes. I think they run wide, but I have a narrow foot. I wouldn't want them any shorter in length though.

 

Got the Sidi Vertebra 2's about a month ago and they seem to be on the big side, I wear a 12 ,euro size 47.

 

I wear a size 11 for New Balance sneakers and Timberland shoes/boots. The Euro sized 45 Veritgo Corsa's fit me perfectly.