Try this:
h = figure; % Create a figure.
h.Visible = 'off'; % Hide it immediately.
h.Units = 'normalized';
% Get original width and height.
originalSize = h.Position
width = originalSize(3)
height = originalSize(4)
% Reassign the size (optional - only if you want to);
width = .5;
height = 0.3; % Whatever you want.
% Get the left and bottom coordinates so we can reposition to there.
xLeft = 0.5 - width/2;
yBottom = 0.5 - height/2;
% Reposition the figure. Here is where it actually gets moved and (optionally) resized.
h.Position = [xLeft, yBottom, width, height]
h.Visible = 'on'; % Now show it.
