There is no good way to do what you want with subplot. Even if you merge multiple plots, there is no control over the size. You probably need to create the axes manually to have fine control over their size. Here's how I might do it.
a=rand(60,120)
b=rand(600,240)
% Create a figure window and set the units to normalized (0=bottom/left, 1=right/top)
h=figure('Units','normalized')
% Divide the figure window into a grid.
% Plot(a) will be between x(2) and x(3), y(11) and y(12)
% Plot(b) will be between x(4) and x(6) - it's twice as wide, y(2) and y(12)
xPos = linspace(0,1,7);
yPos = linspace(0,1,13);
% Place first axis
ax1 = axes('Position',[xPos(2) yPos(11) xPos(2) yPos(2)])
imagesc(ax1,a)
axis equal
axis tight
% Place second axis
ax2 = axes('Position',[xPos(4) yPos(2) xPos(3) yPos(11)])
imagesc(ax2,b)
axis equal
axis tight
