[BTW, is it "Milky Way" or "Milky-Way" ?]
"Milky Way"
What is the width of a spiral arm?
Using http://www.anzwers.org/free/universe/milkyway.html as a reference, roughly 5500 ly
Not only does that page have nice pictures but it's got links to the data to back it up.
To get the 5500 number I measured in pixels across the Sagittarius arm, from the middle of the inter-arm gap on one side to the other. IIRC, the density of stars in the arms vs. gaps is the same, it's just that the arms have a higher concentration of younger, brighter stars.
BTW, the recent versions of Celestia allow you to not only zoom through the solar system and nearby space but all the way to nearby galaxies and look back on the Milky Way. http://www.shatters.net/celestia - Chris Laurel, the author, is an old friend of mine.
Joshua (still lurking, but avoiding politics)
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