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Telescopes are picking up fainter and fainter stars on their edges. As a result, it turns out that many of these galaxies may be a good bit bigger than astronomers had thought -- including our own Milky Way.

One larger galaxy is NGC 300.

Astronomers had pegged its diameter at about 50,000 light-years -- about half the size of the Milky Way. But a few months ago, an international team of astronomers reported that the galaxy is actually twice that size. This extended disk contains lots of faint stars. The number of stars just keeps thinning out as you go farther and farther from the galaxy's core. The team used a giant new telescope in Chile to see the fainter stars.

A few months earlier, another team had reported that the Andromeda galaxy is about three times bigger than previously thought.

The findings suggest that the Milky Way may be bigger than expected, too. It's hard to determine, though, since we're inside the galaxy, so we can't see the whole disk. We have to look through swarms of nearby stars, plus dark clouds of gas, to see its edge.

from http://stardate.org/radio/program.php?f=detail&id=2005-12-08 (http://stardate.org/radio/program.php?f=detail&id=2005-12-08)

Tripling the diameter of spiral galaxies seems pretty significant - the area now expands by a factor of 9, but perhaps the density is similar (or less, or greater?).  So the mass would be several times greater!

And supposedly, the Milky Way is also larger!

With respect to NGC300 -
http://www.physorg.com/news5730.html (http://www.physorg.com/news5730.html)
The research is publishedin the Astrophysical Journal 10 August 2005.

Using the Gemini Multi-Object Spectrograph instrument on the Gemini South telescope in Chile, the observers were able to see stars in the disk up to 47,000 light-years [14.4 kpc] from the galaxy’s centre—double the previously known radius of the disk.

The finding has profound implications for our own Galaxy. Most current estimates put its size at 100,000 light-years across, about the same as the new estimate for NGC 300. “However, our galaxy is much more massive and brighter than NGC 300. So on this basis, our Galaxy is also probably much larger than we previously thought—perhaps as much as 200,000 light-years across,” said the paper’s lead author, Professor Joss Bland-Hawthorn of the Anglo-Australian Observatory.

“We now realize that there are distinctly different types of galaxy disks,” said team member Professor Ken Freeman of the Research School of Astronomy and Astrophysics at the Australian National University. “Probably most truncate—the density of stars in the disk drops off sharply. But NGC 300 just seems to go on forever. The density of stars in the disk falls off very smoothly and gradually.”

The observers traced NGC 300’s disk out to the point where the surface density of stars was equivalent to a one-thousandth of a Sun per square light-year. 

“This is the most extended and diffuse population of stars ever seen,” said Bland-Hawthorn.

So NGC is twice the diameter as previously thought.  So what about other spirals?

Here is another group doing extragalactic stellar astronomy
http://www.sternwarte.uni-erlangen.de/~ai32/research_extra.html (http://www.sternwarte.uni-erlangen.de/~ai32/research_extra.html)
I am just including it here for future reference.