A good size for forum posting is 800 x 600 pixels. Here's an example of a photo at that size...
I believe, but could be wrong, that Photobucket will automatically resize larger pictures to this size when uploaded. The problem with that (assuming it works as stated) is you have to wait for very large pictures to upload. On a high speed connection you might not care but it could become objectionable on lower speed lashups.
Far easier is to resize the pictures before uploading them. There are numerous tools on the web to perform this task. I use PIXresizer from: