Actually the Mother WD was intended to produce the WDs, but it was never shown in the comic, except in the comments section

I wonder if there was any limit to how big a WD could become?

Well, there must be. Things can not just scale up (or down) indefinitely... Considering that it has to have some sort of 'mouth', and can not be of the optimal sphere shape, I would put it's limit to under those of the Death Star.

Still, plenty impressive...

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"Intellectual rigor annoys people because it interferes with the pleasure they derive from allowing their wishes to be the fathers of their thoughts." - George F. Will

"If theory and reality diverges, change reality." - Josef Stalin

Since the WD were meant to land on planets, I'm sure there is a point where the energy requirement to keep the WD afloat on a world would not be worth the effort.

I think the WD were really meant more as teeror weapons than resource collectors. It would make more sense to keep them in space all the time.

According to the Dark Empire rpg source-book I have there isn't one. Maybe there is some sort of phyiscal limit that they would encounter eventaully. But I belive all the ones we saw were "babies" and you would eventaully have gotten them at DS size at least, if not destroyed.

And I know lots of people do not like the rpg, but it does have a say when it doesn't contradict what we know doesn't?