\scriptscriptstyle - Used to over-ride automatic style rules and force scriptscript style.
{ \scriptscriptstyle ...}
\scriptscriptstyle command is used to over-ride automatic style rules and force scriptscript style. Stays in force until the end of math mode or the braced group, or until another style is selected.
\frac ab+\displaystyle\frac ab+\textstyle\frac ab+\scriptstyle\frac ab+\scriptscriptstyle\frac ab
\frac ab + {\scriptscriptstyle \frac cd + \frac ef} + \frac gh
\frac ab + \scriptscriptstyle{\frac cd + \frac ef} + \frac gh