Context :

Aims : Based on our well-determined sample of open clusters in the all-sky catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters , and investigate the effect of mass segregation of stars in open clusters .

Methods : Using statistical methods , we study the distribution of linear sizes as a function of spatial position and cluster age .
We also examine statistically the distribution of stars of different masses within clusters as a function of the cluster age .

Results : No significant dependence of the cluster size on location in the Galaxy is detected for younger clusters ( < 200 Myr ) , whereas older clusters inside the solar orbit turned out to be , on average , smaller than outside .
Also , small old clusters are preferentially found close to the Galactic plane , whereas larger ones more frequently live farther away from the plane and at larger Galactocentric distances .
For clusters with ( V - M _ { V } ) < 10.5 , a clear dependence of the apparent radius on age has been detected : the cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age of 1 Gyr .
A detailed analysis shows that this observed effect can be explained by mass segregation and does not necessarily reflect a real decrease of cluster radii .
We found evidence for the latter for the majority of clusters older than 30 Myr .
Among the youngest clusters ( between 5 and 30 Myr ) , there are some clusters with a significant grade of mass segregation , whereas some others show no segregation at all .
At a cluster age between 50 and 100 Myrs , the distribution of stars of different masses becomes more regular over cluster area .
In older clusters the evolution of the massive stars is the most prominent effect we observe .

Conclusions :