system
Given the following post, respond as if you're commenting on Reddit:
human
Clarification about our position in The Milky Way?

The solar system has been measured to be about 28,000 light years from the center of the galaxy, and about 20 light years above the galaxy's equatorial plane, within the "Orion" spiral arm.  I understand that given what we can see of the Milky Way even with the naked eye, and the knowledge and visual recognition of other spiral galaxies, we can determine the Milky Way is also a spiral galaxy.

1: Without seeing it from the outside, how is it determined that we are out on one of the arms, and how many arms does the Milky Way have?

2: Are all of the stars (yes, stars, not light that is emitted from another galaxy when viewed with a telescope) seen at night contained in our galaxy? 

3a: All images of our galaxy in its entirety are obviously an artists rendition or modeled by a computer, how accurate (from a source like NASA) are these images of the overall shape and size of the Milky Way?

3b: Is it our particular "height" above the equitorial plane that helps give us our estimate of what our galaxy looks like?
gpt
1. Star counts.  Lots and lots of star counts.  Mapping the galaxy is a very difficult, pain-staking, and error-prone process.  The basic idea is that astronomers count the number of stars brighter than some value in various directions.  As you decrease the lowest brightness, you probe farther distances.  This is all confounded by the fact that stars have a large variety of luminosities and (especially) there is a lot of dust in the galaxy which obscures light.  The fact that this dust is somewhat clumpy makes the problem even worse.

As a consequence of the dust, we can't really see much of the other side of the galaxy, particularly toward the galactic center, so we don't really know what's going on there.  There could be a dwarf galaxy hiding behind the Galactic center and we wouldn't know it.  (Although, in principle it would be possible to detect something like that due to its gravitational influence on the Galaxy.)

Another technique is to map neutral hydrogen gas by observing the 21 cm line in radio.  Radio waves don't get obscured by dust, so this can be more useful for mapping out more distant parts of the Galaxy. 

2. No.  It is possible to detect individual stars in other galaxies, though they are the very brightest stars.  In particular, Cepheid stars are visible.  Because their pulsation period is related to their luminosity, they are used to determine the distances to the nearest galaxies.  The main problem with observing these stars is called "blending."  Because the galaxy is so far away, you cannot distinguish a bright Cepheid from dimmer stars that are near it.  The light from all of these stars then hits the same pixel on your detector.  Although the Cepheid usually contributes a substantial fraction of the light, the presence of these other stars makes precise measurements more difficult.

3a. The size is known somewhat well.  There are some quibbles about the distance of the Sun from the center of the Milky Way at about the 10--15% level, but the overall scale is known.  As far as the shape goes, we know that the Milky Way is a barred spiral.  People sometimes argue about the presence of an additional nuclear bar, but it's unknown for certain.  We know that the disk is warped, but again there is controversy over how much and what the shape of the warp looks like.  We know of the existence of a few arms, but nothing is really known about the arms on the other side of the Galaxy. 

3b.  Not really.  We're a little bit above the plane, but not by too much.  Not enough that we would be able to overcome the effects of dust extinction.