Well at 117kg's in a Mega Magic you could surf a 9' which would still be more manageable.
A mate of mine has the 8' and he's about 100kg and it's helping him heaps, on the 9'6" Surftech Robert August WIR he'd take off and the wave would get in front of him. The Walden is shorter but has more volume so the weight dist is better and so without stepping up the board he still get the speed to make sections.
I just think that surfing 117kg's off the tail of a 10' will make the thing plow too much as the weight distribution will be out...
"The Mega Magic is one of the most popular Longboards currently available in the USA. Design inspiration drawn from many larger Tandem and SUP boards.
The volume removed from the length and redistributed it to the width and thickness while maintaining performance. It is about 1/2-3/4" thicker and 1-2" wider than a standard longboard, the Mega Magic will float like a board that is 12-18" longer. For example the bigger guy that normally rides a 10' with the Mega he can ride an 8'-9' without giving up float or stability.
The Mega Magic features the Magic bottom contours and rocker profiles allowing you to push this board to its limits. Riding this board almost feels like cheating because of the unfair advantage you have over others in the lineup"
http://www.surfindustries.com.au/surfboards/walden_megaMagic.php