The circle marker size appears to be the number of points in the diameter. For the other markers, it is not so clear. You can create a square axis with x and y limits of -1 to 1 that is scale inches high and wide
scale = 4;
set(gca, 'Units', 'Inches')
set(gca, 'Position', scale*[0.25, 0.25, 1, 1])
set(gca, 'YTickLabel', [])
set(gca, 'XTickLabel', [])
axis([-1, 1, -1, 1])
axis square
box on
hold on
and then plot all the symbols to be scale*72 units big. I chose 72 since there are 72 points in an inch.
plot(0 , 0, '.', 'MarkerSize', scale*72);
plot(0 , 0, 'o', 'MarkerSize', scale*72);
plot(0 , 0, 'x', 'MarkerSize', scale*72);
plot(0 , 0, '+', 'MarkerSize', scale*72);
plot(0 , 0, '*', 'MarkerSize', scale*72);
plot(0 , 0, 's', 'MarkerSize', scale*72);
plot(0 , 0, 'd', 'MarkerSize', scale*72);
plot(0 , 0, 'v', 'MarkerSize', scale*72);
plot(0 , 0, '^', 'MarkerSize', scale*72);
plot(0 , 0, '<', 'MarkerSize', scale*72);
plot(0 , 0, '>', 'MarkerSize', scale*72);
plot(0 , 0, 'p', 'MarkerSize', scale*72);
plot(0 , 0, 'h', 'MarkerSize', scale*72);
