frcursive package scaling

You can load the fonts at a suitable scale (2/3 seems good).

\documentclass[12pt, a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\DeclareFontFamily{T1}{frc}{}
\DeclareFontShape{T1}{frc}{m}{n}{%
  <-8>   s*[0.6667] frcr6
  <8-14> s*[0.6667] frcr10
  <14->  s*[0.6667] frcr14
}{}
\DeclareFontShape{T1}{frc}{m}{sl}{%
  <-8>   s*[0.6667] frcsl6
  <8-14> s*[0.6667] frcsl10
  <14->  s*[0.6667] frcsl14
}{}
\DeclareFontShape{T1}{frc}{m}{it}{<-> ssub*frc/m/sl}{}
\DeclareFontShape{T1}{frc}{bx}{n}{%
  <-8>   s*[0.6667] frcbx6
  <8-14> s*[0.6667] frcbx10
  <14->  s*[0.6667] frcbx14
}{}
\DeclareFontShape{T1}{frc}{bx}{sl}{%
  <-8>   s*[0.6667] frcbxsl6
  <8-14> s*[0.6667] frcbxsl10
  <14->  s*[0.6667] frcbxsl14
}{}
\DeclareFontShape{T1}{frc}{bx}{it}{<-> ssub*frc/bx/sl}{}

\DeclareRobustCommand{\cursive}{\fontfamily{frc}\selectfont}
\DeclareTextFontCommand{\textcursive}{\cursive}

\begin{document}

\textcursive{ABCD}A

\textit{\textcursive{ABCD}A}

\textbf{\textcursive{ABCD}A}

\end{document}

About scaling only the uppercase letters, I don't think that it can give a nice result: it's pretty ugly, to be honest:

\documentclass[12pt, a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage{frcursive}
\makeatletter
\renewenvironment{cursive}{%
  \fontencoding{\frcursive@enc}%
  \fontfamily{frc}%
  \fontsize{9pt}{10pt}%
  \linespread{1.5}%
  \selectfont}{\par}
 \makeatother
\begin{document}
A \begin{cursive} A \end{cursive}
\end{document}