I generally just post pics to the gallery at my screen resolution, which is usually quite a bit bigger than most posts.
Some people don't like scrolling an image and others apologise for their large image size.
Personally, I don't like small images in the galleries, but that's just me.

Is there an accepted size which would please most people?
I'm not interested in how big you render, just upload size.

I think people just do whatever they like. Personally, I use a high definition LCD TV screen as a computer monitor and so I appreciate when an image is large. It’s disappointing when a great image is too small.

I too use my HD LCD TV as a monitor, it's a 32" one & I think it's great to see pictures which fill the screen. My latest upload is 1330 x 930 which I think is about right. I don't mind scrolling an image as I like to see the detail, I've seen too many small uploads here which don't do the picture justice.

I specify full size when I post, and often that's quite large (1600x1200) - however the user has control over that in their viewing preferences.

I think 1024x768 is a decent sized image and one that most people can view comfortably on their monitors.

A friend of mine  has a high resolution monitor but doesn't like working with a large poser scene window so she works in the default size.  She always rendered at that same size so her images were always pretty small.  She got several comments in her gallery from people telling her that she should make them larger because they were hard to see.

I had been telling her the same thing for months but she didn't listen until she heard it from several others.

The first image she rendered at a larger size was something like 4000x4000 or whatever the max size in the gallery is.  When she sent me the link to view her newest image all I saw was an eye of some animal. In order for me to see that full image I would have had to scroll up and down and left and right.  I didn't bother. I sent her a note back saying "Nice eye!" and that her image was way too big for the average person to view. Now she renders at either 800x800 or 1024x768

I see no reason to upload an image that is 4000x4000 pixels in size.   I have a 22" high resolution monitor and even I can't view something that large without scrolling all over the place.  Most people don't have IMAX screens in their home :)

Yup- 1024 is a good compromise when posting.

As an artist YOU may allways get the latest and greatest new tool. But most people don't.

Between 800 and 1200 are what most use. So 1024 is a good posting size compromise.

Also, it is an error to assume that everyone has access to higher speed internet connectios. A HUGE number live in underserved areas where dial up is the best they can get. So massive file sizes kinda suck.

900x1500 or 1441x990 or thereabouts normally.  Sometimes it's 1024x768 - depends on what I'm rendering really

Its around 1024X 768 for me too.  Additionally, 500KB is plenty for both viewing and printing on standard photo paper (8 X 10.5 print).   Results are excellent.  Plutom

Personally, I tend to agree with geoegress on this one.

I think consideration for the general viewing/downloading public is the main thing to keep in mind. Most do enjoy viewing images that fill their screen without excessive scrolling and of course there will be some who have huge monitors while others do not.

Also, yes it's true, there are some still using dialup modems. (more than one may think actually) so loading time is another consideration. 

I think while some are indeed moving to larger monitors, I believe the "MAJORITY" of present day computer users still have traditional 19" - 22" range screens, not to mention a lot of people on laptops.

For gallery uploads, I tend to upload at 1024 because I think it falls in a range that is better suited to  the average user.

Sure if I render something for a specific purpose that requires a higher resolution, I render to suit the need, but for standard web use, I prefer to consider what the majority of viewers would most likely be viewing in.

I am a dialup user, I tend to avoid images that take ages to load up, much as I'd like to view them. :(

i think it kinda depends on the subject matter. Portraits dont need to be that big to see them well. But the more detailed an image is the more i want to see a large render to appreciate all the detail. Unfortunately some of my favorite Poser artists dont post very large images for fear of them being ripped off, as has happened before.