Getting the right frame size is much easier, now that most brands publish the stack and reach. Those two values tell the story. It makes no difference what the size number is.
The only other thing to pay attention to is the seat tube angle. I ride smaller sizes that nearly always have a 74 to 74.5 degree STA and either one will work with a 25mm setback post. If the bike has a steeper 75 degree STA, I'd need a 32mm setback and I avoid that.