Both the Borg Cube (seen in "Q Who") and the Crystalline Entity (seen in "Silicon Avatar") 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.
MAGNUS: I've got a visual.
ERIN: It's massive. Twenty eight cubic kilometres, a hundred and twenty nine thousand lifeforms on board. We're being scanned. They haven't altered course.
Voy : Dark Frontier
This should give you a rough idea of scale: