Both the Borg Cube (seen in Q-Who) and the Crystalline Entity (seen in Silicon Avatar) seem to dwarf the Enterprise-D.
Is it possible to know which object is larger? I haven't been able to find the dimensions of the Crystalline Entity anywhere.
The Crystalline entity is very much smaller than a standard (Type-2) Borg Cube.
The Star Trek Encylopedia describes the Crystalline Entity as being "hundreds of meters across", versus the Borg Cube which is over 3 kilometres (3040m) along each side.
This should give you a rough idea of scale: