The option to resize an image is found in the Image Editor, which is available for use when you are logged in to your Photobucket account.
First, get to the Image Editor.
An easy way to get to there is via the link at the top of the page. Select the Edit tab. That will bring up a window with thumbnail views of the images within the album you have selected.
Next, click on the image you would like to edit.
After you have selected a thumbnail image to edit, that image will be opened in the editor to display in full (very large images may be zoomed out to display the entire image). When viewing the editor, you will see the Resize option in the middle of the editing options bar, located above the image.
Selecting the Resize option will bring up sizing options for the image. Changing the size will change both the width and height of the image, as the option to maintain proportions is automatically selected.
To change just one parameter for the image size (width or height), select the lock icon in between the width and height, and that will then allow you to change just one sizing option without affecting the other.
Select Apply, and the new sizes will be applied to the image within the editor screen. Select save, and that image will then be resized to your new size settings, and will appear in your account at that size. Please note that saving and replacing the image will save over the original size image you have uploaded, and that original size image will no longer exist.