convolution matrix

Say i have image with size m*m , and convert it to a vector with size m^2*1 and i have an average filter , f=imread('lena.jpg'); f=im2double(rgb2gray(f)); [r c]=size(f); f=reshape(f',r*c,1); h = ones(4,4)/16;

my question is how to make this filter with size m^2*m^2 i use : h=convmtx(h,r*c); but there is an error with me say ::::: Maximum variable size allowed by the program is exceeded.

Error in ==> convmtx at 26 t = cidx(:,ones(n,1)) + ridx(ones(m,1),:); % Toeplitz subscripts