In 5.6 and 5.7, it is allowed to set this value to 0 or other some other extremely small value.  This is pretty much an invalid configuration.  In most observable cases, it seems that TokuDB needs at least a few thousand bytes in order to function at a most basic level.  Lets set the minimum allowable value to ~128K for any setting < than ~128K except for 0 which indicates to autotune based on the size of the cachetable.