Very noob question
 

how to you do the measure thing from where to where. 51cm, 52 cm,53cm,54cm,55cm etc thank you

center to center is normal, many do center to top now that compact geo is the norm.

it's center of bottom bracket to center of top tube/seattube joint... or top of seattube, or "effective top tube" where the imaginary line of the headtube top tube joins.

if this confuses you, wikipedia it.