Here's a cute little function that does what you're looking for.
function textWithZoomEffect
% plot some original data
t = linspace(0,2*pi);
x = sin(t);
plot(t,x)
% get axes size
axis([0,2*pi,-1.5,1.5]);
ax = axis;
% add some text
txt = text(pi/2,1,'Max','fontsize',40/(ax(4)-ax(3)));
h = zoom; % get handle to zoom utility
set(h,'ActionPostCallback',@zoomCallBack);
set(h,'Enable','on');
% everytime you zoom in, this function is executed
function zoomCallBack(~, evd)
% Since i expect to zoom in ax(4)-ax(3) gets smaller, so fontsize
% gets bigger.
ax = axis(evd.Axes); % get axis size
% change font size accordingly
set(txt,'FontSize',40/(ax(4)-ax(3)));
end
end
