G = NaN(250,250);
    a = ceil(rand(1,50)*250*250);
    b = ceil(rand(1,50)*250*250);
    G (a) = 1; G (b) = 0;

% find the non-NaN entries in G
idx = ~isnan(G);

% find their corresponding row/column indices
[i,j] = find(idx);

% resize your matrix as desired, i.e. scale the row/column indices
i = ceil(i*100/250);
j = ceil(j*100/250);

% write the old non-NaN entries to Gnew using accumarray
% you have to set the correct size of Gnew explicitly
% maximum value is chosen if many entries share the same scaled i/j indices
% NaNs are used as the fill
Gnew = accumarray([i, j], G(idx), [100 100], @max, NaN);