Without addressing the specifics of floatrow, about which I know nothing, your problem has two components: scaling the figures to the same height, and scaling the overall width to some dimension, such as \textwidth. This MWE shows how to do both, using the scalerel package to match the height of one object to the next (while preserving the aspect ratio). Then, I do a scale on the result to match the width to, in this case, \textwidth.
Where my figures are composed of \rules, yours would be composed of tikz images. Note also that \scalerel assumes its arguments are in math mode, and so I include the math delimiters to, in essence, get out of math mode.
I placed all the figures side-by-side into a single box \x. However, if your requirements (for captioning, etc.) required you to keep the images as separate entities, \scalerel*{$\figa$}{$\figd$} would be the command to create a version of just \figa scaled to the size of \figd. You of course, would still have to \scalebox it by a factor of \scaleratio to get it to the desired final size (I show this case as my final example).
EDITED to be compatible with latest package (scalerel V1.5)
\documentclass{article}
\usepackage{scalerel}
\usepackage{fp}
\usepackage{stackengine}
\def\stacktype{L}
\usepackage{xcolor}
\begin{document}
\def\figa{\color{blue}\rule{.5in}{.7in}}
\def\figb{\color{red}\rule{.7in}{.3in}}
\def\figc{\color{green}\rule{.4in}{.5in}}
\def\figd{\color{black}\rule{1.in}{.8in}}
Figures as is:
\figa\figb\figc\figd
figures scaled to height of figd
\newsavebox\x
\sbox\x{\scalerel{$\figc$}{$\figd$}}
\sbox\x{\scalerel{$\figb$}{\usebox{\x}}}
\sbox\x{\scalerel{$\figa$}{\usebox{\x}}}
\usebox{\x}
textwidth rule
\rule[1ex]{\textwidth}{.1ex}
figures scaled to textwidth in one box
\newcount\figwidthc
\newcount\textwidthc
\figwidthc=\wd\x
\textwidthc=\textwidth
\FPdiv\scaleratio{\the\textwidthc}{\the\figwidthc}
\scalebox{\scaleratio}{\usebox{\x}}
individual figures scaled to textwidth
\stackunder{\scalebox{\scaleratio}{\scalerel*{$\figa$}{$\figd$}}}{(a)}%
\stackunder{\scalebox{\scaleratio}{\scalerel*{$\figb$}{$\figd$}}}{(b)}%
\stackunder{\scalebox{\scaleratio}{\scalerel*{$\figc$}{$\figd$}}}{(c)}%
\stackunder{\scalebox{\scaleratio}{\figd}}{(d)}
\end{document}

scalerelpackage (1.5). When I wrote my original answer (not from my usual workplace), I apparently had an older version. The type of nesting I did in my\sboxsyntax no longer worked. I have revised my answer to reflect this, and the fact that you didn't complain about it must mean you have an older version too. Please update to V1.5 – Steven B. Segletes Jul 31 '13 at 15:45