I would just add an extra arrow of given length to the vector field:
% random array of vectors
x=[1:6]; y=[1:5];
u = rand(numel(y),numel(x));
v = rand(numel(y),numel(x));
% create an extra entry for the reference arrow
x = [x max(x)+1];
u = [u NaN(numel(y),1)]; u(1,end) = 1;
v = [v NaN(numel(y),1)]; v(1,end) = 0;
figure
quiver(x,y,u,v)
text(max(x),1,'1 m/s','verticalalignment','bottom')
