Not that big. Advanced technology, like replicators and a few million years are enough to reach galaxies far-far away.
Everything that is reliable for a couple of million of years will be heavy. If it is heavy - it will be slow. And we have that joyous dark energy lurking in the intergalactic space - traversing it may not be the easiest thing in the world.
I thought dark energy is something that doesn't interact with ordinary matter.
Except through gravity
Big relative to our tech. How long ago was it that we thought Earth-like planets were extremely rare? 20 years? 10?