If you are using an up to date version of Matlab (scanning the release notes I think R2019a or later for the particular syntax of min used here) this should work, although I don't have time to fully test it:
a = rand(17,1);
b = rand(300);
[~, idx] = min( abs( b - reshape( a, 1, 1, [] ) ), [], [1 2], 'linear' );
[row,col] = ind2sub( [300, 300], squeeze( idx ) );
