It'd be easier if there weren't different variables, but since they're inconsistent in length that makes it a little more of a pain unless you were to initially use a cell array where each cell column contained one of these arrays.
But, the syntax you're looking for to do the size reduction is
[mn,imn]=min(length(A),length(B),...
A(N+1:end)=[];
B(N+1:end)=[];
C(N+1:end)=[];
...
Alternatively, you can save instead of cull...
A=A(1:N);
B=B(1:N);
C=C(1:N);
...
this way you don't need to worry about which doesn't have >N elements.
Of course, going on into the problem, you'll undoubtedly want to end up with
Data=[A(1:N) B(1:N) C(1:N) ...
so you can now programmatically address all by simply using subscripting expressions instead of having superfluous variables to deal with.
How were these arrays created in the first place? Quite possibly you could have solved all the issues while reading them or some other way before got into this fix.