It is the opposite! Quicksaves and autosaves cause save bloat! Avoid auto saving and quicksaving ever at all cost! Too many quick/auto saves can leave a save permanently bloated and unloadable!
Delete your data folder then use the repair function in steam. You can't magically have a letterbox, there is a file that is doing this. That or you have the wrong resolution set.
Next time read the mod description. It has a list of all the incorporated changes and fixes. It for example includes this fix: https://www.nexusmods.com/skyrim/mods/76607?tab=files