Is there anyway to reduce the font size less than scriptsize in subcaption?

Question

I want to use four picture on one slide, using the subfigure command. I did it, but the font size of the subcption is really big, I'm using scriptsize compared with four subfig. Is the anyway to reduce it less than scriptsize , I found some suggestions here but doesn't help. When I tried using tiny I got the following error:

Package caption Error: tiny undefined. \end{frame}. 

How to figure out this. My code is here:

\documentclass{beamer}
\usepackage{beamerthemesplit}
\usepackage{tikz}
\DeclareGraphicsExtensions{.pdf,.png,.jpg,.eps}
\usepackage{epsfig}
\usepackage{epstopdf}
\usepackage{tcolorbox}
\graphicspath{{./figs/}}
% for 2nd questoin
\usepackage[labelfont=bf,textfont=scriptsize]{caption}
\usepackage[labelfont=scriptsize,textfont=scriptsize,singlelinecheck=on]{subcaption} % will apply to all subcaptions
% set up labelformat and labelsep for figure
\captionsetup[figure]{labelformat=empty, labelsep=none}
% set up labelformat and labelsep for subfigure
\captionsetup[subfigure]{labelformat=parens, labelsep=none}
\renewcommand{\thesubfigure}{\roman{subfigure}}
\captionsetup{compatibility=false}
\usepackage{anyfontsize} 
 \begin{document} \begin{frame}{FBL Trajectory Tracking control}

\vspace{-0.3cm}
\begin{figure}
    \centering
    \begin{subfigure}[b]{0.48\textwidth}
        \vspace{-0.7\baselineskip}
        \includegraphics[width=1.1\textwidth,height=0.385\textheight]{FBLPT1}
        %           \vspace{-1.07\baselineskip}
        \caption{Joint 1 torque}
        \label{fig:FBLPT1}
    \end{subfigure}
    ~
    \begin{subfigure}[b]{0.48\textwidth}
        \vspace{-0.7\baselineskip}      
        \includegraphics[width=1.1\textwidth,height=0.385\textheight]{FBLPT2}
        %           \vspace{-1.07\baselineskip}
        \caption{Joint 2 torque}
        \label{fig:FBLPT2}
    \end{subfigure}
    %\vspace{-2.07\baselineskip}
    \begin{subfigure}[b]{0.48\textwidth}
        \vspace{-0.7\baselineskip}
        \includegraphics[width=1.1\textwidth,height=0.385\textheight]{FBLPT3}
        \caption{Joint 3 torque}
        \label{fig:FBLPT3}
    \end{subfigure}
    ~
    \begin{subfigure}[b]{0.48\textwidth}
        \vspace{-0.7\baselineskip}      
        \includegraphics[width=1.1\textwidth,height=0.385\textheight]{FBLPT4}
        \caption{Joint 4 torque}
        \label{fig:FBLPT4}
    \end{subfigure}         
\end{figure}
\end{frame}
\end{document}