How to get a smaller plane from a bigger plane?

t=(0:90:360)';
SourcePlaneCircle=[1.*cosd(t) 1.*sind(t) zeros(length(t),1)];
r=vrrotvec2mat(vrrotvec([0 0 1],[1 0 0]));               %Calculate rotation between two vectors
circle=SourcePlaneCircle*r'+repmat([1 0 0],length(SourcePlaneCircle),1);
SourcePlane=patch(circle(:,1),circle(:,2),circle(:,3),.5);

for the code above, how do I get a smaller plane from the "SourcePlane"?

Because I want to get the coordinates of the smaller plane (in yellow) from the one in blue. Therefore how do I plot the smaller plane out?

I know that the function rotate in matlab can work. But because i need to flexible my plane in any direction therefore rotate might not suit.

[KSSV]

You can pick a sub part from the main part in many ways..what is your criteria?

[tan]

My criteria is to generate a square plane that can be faced in any direction.