documentclass{article}
usepackage{tabu}
%method 1:
newlength{mylength}
settoheight{mylength}{%
begin{tabu}{|c|c|}
test & test\
test & test\
test & test\
test & test
end{tabu}
}

%method 2:
newcommand{testtbl}{%
begin{tabu}{|c|c|}
test & test\
test & test\
test & test\
test & test
end{tabu}
}
newcommand{ntabht}{%
  setbox0=hbox{testtbl} theht0%
}
begin{document}

themylength

ntabht
end{document}