Keep in mind that size wise for posting is defined in bytes while the size we're interested here is dimension.
You can have a tiny pic with 100x100 pixel filling space wise 1Gb if you set the dpi very high. That is useless in terms of internet.
like Maureen says 800x600 pixels is a great size for internet view, but remember to set your dpi to 72 (internet quality) or about 90 (max 150!).
But if you only want to show the pics on the internet and not print them, then 72 is fine.
Also when saving your pic after re-sizing, you can adjust the level of compression... if you set it to 80 or 75% quality you'll still end up with a good enough picture but it'll take less physical space (in other words, keep the file small, even though the picture is big
).
Can't remember how big files can be to be posted here, but if you keep the file size to under 100Kb you should be fine.
Play with the settings until your satisfied with the picture size/quality and file size that looks right.
If it all sounds very complicated, there are programs and websites that can do all that for you automatically.
There are those who believe that life here began out there...