Do you want the first 1000 elements or a 1k X 1k array as the title says? They're not commensurate.
Either way; pick the N that you want and then reshape
Say it's a 1K by 1K...
x=load('yourmatfile');
N=1000; % set the size desired for generality
x=reshape(x(1:N*N),N,N); % keep the N^2 first and reshape to square..
ADDENDUM:
x=load('yourmatfile');
N=1000; % set the size desired for generality
N=min(N,sqrt(length(x)); % modify to make fit if length(x)<N^2
x=reshape(x(1:N*N),N,N); % keep the N^2 first and reshape to square..
That'll fix the overrun error as also noted elsewhere but gets the solution into the Answers section...
