A construct to enable, centered, cropped-to-fit content to be generated dynamically. Useful for equations/figures etc. This example uses TeXShade for sequence alignment.

crop_to_fix.tex

\documentclass[a4paper, oneside, 11pt]{memoir} % Doc Setup
\usepackage[left=1cm, right=1cm, top=1cm, bottom=1cm]{geometry}  % Doc Geometry

\usepackage{texshade}
\usepackage{lscape}
\usepackage[active, tightpage]{preview}

\begin{document}
\begin{landscape}
\begin{preview}
\begin{vplace}
    \begin{texshade}{/path/to/input.aln}
        \seqtype{P}
        \alignment{center}
        \setfont{numbering}{tt}{md}{sc}{small}
        \setfont{names}{tt}{md}{up}{tiny}
        \setfont{residues}{tt}{md}{sc}{tiny}
        \setfont{ruler}{tt}{bf}{sc}{tiny}
        \shadingmode[allmatchspecial]{similar}
        \smallblockskip
        \feature{top}{1}{25..25}{fill:$\downarrow$}{Hi, I'm an annotation}
        \feature{bottom}{1}{70..76}{brace}{So am I!}
    \end{texshade}
\end{vplace}
\end{preview}
\end{landscape}
\end{document}