It's all personal preference. I'm 5'10" exactly and around 165 lbs. I have typically bought size medium stuff from Castelli. It's definitely not "American" fit (aka, "club" fit or "fondo" fit), it's a traditional race fit. I prefer that traditional race fit, personally. The same is true of brands like Giordana, Assos, Santini, Sportful, Bioracer, Eliel, Ornot, MAAP, Pedal Mafia etc in my experience: all offer a race fit. I think Castelli gets singled out because it's more readily available than many of those other brands, but they're not unusually tight when compared to these similar "race focused" brands in my experience. Bigger brands in the US like Pearl Izumi, Bontrager, Specialized, Garneau, Bellweather, Voler etc generally sell much looser fitting stuff, with exceptions at the top end or "race" collections.