Technically, there is no size limit for boats operating on the Lake of the Ozarks, but there are limitations to dock slip size.
"The maximum boat slip size shall not exceed 55 feet in length for residential permits and
60 feet for commercial (marina) permits. Any watercraft moored within slips permitted
after March 30, 2008 shall not extend beyond the mouth of the slip." - SMP: Appendix B (2007)
The slip size restriction was set in place as a broadside effort to limit the size of boats on the lake. The March 30th, 2008 date insures that as docks are replaced, or require re-permitting for any reason, their slips will be within the maximum size and no boat extends beyond the roof-line. Slip size restrictions do not imply restrictions to the overall size of the dock, which can be quite large and limited only by the same rules as any other dock such as property line extensions, set-back minimums, cove "head" limitations, and how much one is willing to spend each year for a permit. Any dock over 3000 sq/ft is assessed an extra six cents per square foot annually, rather than just a one time fee.
Some might think that using a pier is a way to get around the limitation but Ameren has rules which make this option difficult and ultimately no advantage for a larger boat.
"Piers must be 6 feet or less in width, and must not extend more than 30 feet lakeward of elevation 658.5 feet." -SMP: Appendix B (2007)
For property where a pier is even practical, let alone possible, the pier length limit of 30 feet does not facilitate mooring large boats. A 70 foot boat is not going to tie up to a 30 ft pier and be secure, although some configuration might be possible.
There are exceptions of course, the obvious being the motor vessel "Celebration", and it's sister ship the "Celebration II", 80 foot excursion boats popular for daytime and dinner cruises. It is unclear what rules govern commercial vessel mooring facilities, but for private residences the only means of having a dock large enough to accommodate a 60+ foot boat is to already have had it built and permitted over six years ago. As docks age and are replaced, these will become rarer and ultimately the slip size restriction will start to have an affect.