Is there different types of mollies on the basis of size?

[Aniket]

I have 5-6 cm mollies and i have seen 10-11cm gold dust mollies in pet shop. Are they different or only matter of age?

 

[NickAu]

Are they different or only matter of age?

A matter of age and the type, female mollies can grow to a maximum size of 6 inches (15 cm)

 

[Colin_T]

Possibly different species with the bigger ones probably being sailfin mollies (Poecilia velifera or latipinna) and the smaller ones most likely Poecilia sphenops.

The sailfin mollies have a much bigger dorsal fin and grow to twice the size of the sphenops.

They both come in the same colours and were hybridises years ago. But you can sometimes buy pure sailfins.