there is no definitive answer for you it all depends on what type of picture you're putting up on the site.
When you are in the post page and you click on manage attachments a window will show up and you'll need to scroll down and it will tell you the maximum size for you.
Keep in mind that for pictures (jpg, gif, png) there are limits for file size and dimensions.
a good almost foolproof resizer is called
pixresizer it works well and will give you the pictures that are small enough in both size and image dimensions.