\documentclass{article}
\usepackage{datatool}
\usepackage[strict]{changepage}
\usepackage{filecontents}
\usepackage{tcolorbox}
\usepackage[absolute,overlay]{textpos}
\usepackage{xcolor}
\usepackage{xparse}

\begin{filecontents*}{test1.csv}
Acol, Bcol,Ccol
Ax,Bx,1
Ay,By,3
A1,B22,1
A2,B44,2
A3,B11,3
\end{filecontents*}
% If you include a line such as 'A4,B55,4' without saying how \xLength and
% \yLength should be set in this case, you'll get the error message:
%
%   Package lforti Error: Encountered value '4', but no case was written for it.

\DTLloaddb{mydata1}{test1.csv}

\newlength{\xlengthForOddPages}
\newlength{\xlengthForEvenPages}
\newlength{\ylengthForOddPages}
\newlength{\ylengthForEvenPages}

\setlength{\xlengthForOddPages}{2cm}
\setlength{\xlengthForEvenPages}{10cm}
\setlength{\ylengthForOddPages}{1.5cm}
\setlength{\ylengthForEvenPages}{5cm}

\newif\ifmyoddpage              % always set globally, contrary to \ifoddpage

\newcommand*{\xpageDependent}{%
  \ifmyoddpage
    \expandafter
    \xlengthForOddPages
  \else
    \expandafter
    \xlengthForEvenPages
  \fi
}

\newcommand*{\ypageDependent}{%
  \ifmyoddpage
    \expandafter
    \ylengthForOddPages
  \else
    \expandafter
    \ylengthForEvenPages
  \fi
}

\newcommand*{\mycheckoddpage}{%
  \checkoddpage
  \global\let\ifmyoddpage=\ifoddpage
}

\ExplSyntaxOn

\msg_new:nnn { lforti } { unhandled-value }
  { Encountered~value~'\exp_not:n~{#1}',~but~no~case~was~written~for~it. }

\tl_new:N \l__lforti_cond_length_setup_conds_tl
\seq_new:N \l__lforti_cond_length_setup_tmpa_seq
\seq_new:N \l__lforti_cond_length_setup_tmpb_seq

% Flexible length setup that may depend on whether it is called on an odd page
% or on an even page.
%
% #1: a LaTeX length command (or a TeX \skipdef token) to set according to
%     #2 and #3
% #2: the value to test against
% #3: a comma list describing the different cases:
%
%       {value_1, length-for-odd-pages, length-for-even-pages},
%       {value_2, length-for-odd-pages, length-for-even-pages},
%       ...
%       {value_n, length-for-odd-pages, length-for-even-pages},
\cs_new_protected:Npn \lforti_cond_length_setup:Nnn #1#2#3
  {
    \checkoddpage               % make sure it is used on the relevant page
    \seq_set_from_clist:Nn \l__lforti_cond_length_setup_tmpa_seq {#3}
    \tl_clear:N \l__lforti_cond_length_setup_conds_tl

    \seq_map_inline:Nn \l__lforti_cond_length_setup_tmpa_seq
      {
        \seq_set_from_clist:Nn \l__lforti_cond_length_setup_tmpb_seq {##1}
        \tl_put_right:Nx \l__lforti_cond_length_setup_conds_tl
          {
            % Value to check against
            { \seq_item:Nn \l__lforti_cond_length_setup_tmpb_seq { 1 } }
            { % Corresponding setup code for #1
              \exp_not:N \skip_set:Nn \exp_not:N #1
                {
                  \seq_item:Nn
                    \l__lforti_cond_length_setup_tmpb_seq
                    { \ifoddpage 2 \else 3 \fi }
                }
            }
          }
      }

    % Handle value #2 according to the cases described by #3
    \str_case:nVF {#2} \l__lforti_cond_length_setup_conds_tl
      { \msg_error:nnn { lforti } { unhandled-value } {#2} }
  }

\cs_generate_variant:Nn \lforti_cond_length_setup:Nnn { Nx }

% Same as \lforti_cond_length_setup:Nnn (see above), except that the second
% argument will be fully expanded (\lforti_cond_length_setup:Nnn will only see
% the *result* of this expansion in its #2).
\NewDocumentCommand \lforticondlengthsetup { m m m }
  {
    \lforti_cond_length_setup:Nxn #1 {#2} {#3}
  }

\ExplSyntaxOff

\newlength{\xLength}
\newlength{\yLength}


\begin{document}
\lforticondlengthsetup{\xLength}{\C}{%
  {1, 2cm, 4cm}, % When \C expands to 1, use 2cm on odd pages, 4cm on even pages
  {2, 3cm, 5cm}, % When \C expands to 2, use 3cm on odd pages, 5cm on even pages
  {3, 4cm, 4cm}, % When \C expands to 3, use 4cm on all pages
}%
\lforticondlengthsetup{\yLength}{\C}{% analogous setup for \yLength
  {1, 1.5cm, 12cm},
  {2, 2cm, 23cm},
  {3, 3cm, 1.5cm},
}%
\DTLforeach*{mydata1}{\A=Acol,\B=Bcol,\C=Ccol}%
{%
\begin{textblock*}{5cm}(\xLength,\yLength)
  \color{blue!50!black}%
  \noindent
  abc \\
  \texttt{\string\xLength} = \the\xLength\\
  \texttt{\string\yLength} = \the\yLength
\end{textblock*}

\begin{tcolorbox}[width=\xLength,height=\yLength,
                  title={Title}]
  \C
\end{tcolorbox}

\newpage
}

\end{document}