Hopefully this might help. There are two ways to control the image size.
Auto Resize when you upload the image to Imgur and an image size
selector when you post the link to the forum.
When uploading to Imgur, select the Auto Resize from the drop
down menu on the right side of the screen before you select the
pictures to upload.
If you're creating the image link one at a time from this screen, note
that at the lower RH corner there's a choice of sizes. Choose
"Huge Thumbnail", then copy the BBCode.
If using this screen for multiple pictures, use the pull down menu
"original Image" and select "Huge Thumbnail"
Note also that you can edit the post on the forum above where the
pictures are oversize by adding the letter h immediately ahead
of the .jpg and that converts the size to Huge Thumbnail.