It could be changed by using 'ItemTokenSize' parameter. Please refer to the sample code below:
% Create a bar graph
x = 1:5;
y = [10 15 8 12 9];
bar(x, y);
% Get the handle to the legend object
legendHandle = legend('Data');
% Set the desired length of the bar in the legend axes
desiredLength = 20; % Modify this value as needed
% Set the desired height of the bar in the legend axes
desiredHeight = 10; % Modify this value as needed
% Adjust the ItemTokenSize property of the legend
set(legendHandle, 'ItemTokenSize', [desiredLength desiredHeight]);
