After the smoke clears - size is next

Hi Everybody,
    After the smoke clears,  we need a discussion of the size at which images, text, and the forum page itself are displayed in this forum.  I had it under control in the newsgroups.  Now it's a total muddle to me.
    The problem arises because Windows gives the user a bewildering number of ways to control size.  It has been a source of confusion for most people since the beginning of time. Just to kick off a future discussion, here are the ones that I am familiar with.
1.  Screen resolution.  It's best to use the maximum or there may be severe distortions.
2.  "Display Size" as found in Control Panel. This does not change the resolution.  It up-samples  text  and some images.  I use 150%.  That increases the sensation of size by over 200%.  Very nice  for the Curves dialog in PhotoPaint.
3. "Window Color and Appearance" as found in Control Panel. You get to pick the size, text, and color of about twenty items -- such as a menu bar.  Very confusing at first but a great asset once you catch on.
4. Next in line, if you use Internet Explorer you can vary the size of text and images (zoom level) in Internet Explorer.
       NOW COMES THE MUDDLE FOR ME.
1.  What in the world does this site use to set the size of a JPEG image which I post ??  How high on the monitor is an image which is 700 pixels high in the file
2.  How in the world do I set my own size settings so that most people will see my posted masterpiece without having to change settings.
    Hoping to hear from you when the dust settles here.
Phil
PS:  In the newsgroups, an image always displayed at "actual" size: i.e. the pixel size on the monitor was exactly the same as pixel size in the file.  That was very nice.  You knew exactly how it would appear on someone else's monitor.