Kam,
On my screens, it takes about 100 px on the long side to fill the CiC frame. See, for example,
Black-eyed Susans.
If you paste a link to something larger than that, it will show up in larger form if people double click on the image. If I want to get comments on details, I typically post a link that is 2000 px or so on the long side.
Simply pasting the link will take people to your flickr page, but it won't insert the image here. You have to use the image icon, the third from the right above the box where you type your message. Smugmug produces the correct URL for this purpose. I think that to use Flicker, you may need to remove BB code from the link before you paste it, but I don't use Flickr, so I don't know. Many people here do, and some have posted explicit instructions.
Can a Flickr user help Karm with this?