Question

How big can a nebula become?

Answer 1

Nebulae are huge interstellar clouds of gas and dust that range in size from 1 AU to10 AU, and which are less dense than the deepest vacuum in Earth's laboratories. Just one portion of of a nebula the size of Earth would weigh only a few kilograms!

Answer 2

Here's the gist of my answer, for simplicity:

The largest nebulae are HII regions, clouds of gas ionized by young hot stars forming inside them.

We can calculate the radius of a sphere corresponding to the maximum distance at which neutral hydrogen gas can be ionized - a proxy for the size of the HII region.

This method can be adapted for clusters of stars, not just individual ones.

Basic assumptions about the masses of molecular clouds and the star-forming efficiency show that the maximum size of an HII region should be about 2150 light-years. This is a couple times the size of the largest known HII regions.

Essentially, yes, you can have extremely large nebulae that would take a long time to cross, even at exceptionally high speeds.

Large nebulae are HII regions

If you look at some of the largest nebulae currently known, you might notice that many of them, measuring hundreds of light-years in diameter, are HII regions. They're are stellar cradles, clouds of hydrogen ionized by the young, newly-formed stars inside them. Their evolution is governed by the emission from the hottest massive stars that provide the ionizing radiation, and will eventually disperse the clouds entirely. HII regions are good choices for large nebulae simply because they're extremely massive, and may contain dozens of stars.

Many of the largest nebulae are HII regions:

The Tarantula Nebula

The Carina Nebula

NGC 604

HII regions aren't always the sites of starbirth; they can form (at smaller scales) around single stars. Barnard's Loop is a famous example of a large HII region that is thought to have formed from a supernova. However, the very largest HII regions are indeed these descendents of molecular clouds, containing clusters of young stars.

Strömgren spheres comes second