axesm projection, frame is too small

I have a plot of the english channel. It's maplonlim is between 0 and 5 degrees. By setting these limits the plot is very small, how can I set the size of the plot?

h=figure('color','w','Position', [20 20 1000 800]);
sp1=subplot(2,1,1);
axesm('mapproj','mercator',...
           'maplatlim',[51 53],'maplonlim',[0 5],'MLineLocation',[0 2 4],'PLineLocation',[51 52 53],'FFill',50);
axis off, gridm on, framem on;