isn't a matlab command, it's a function which was propsed by gonzalez in his book "Image Processing using Matlab", the source code of it is given below
function PQ = paddedsize(AB, CD, PARAM)
if nargin ==1
      PQ = 2*AB;
elseif nargin == 2 & ~ischar(CD)
      PQ = AB +CD -1;
      PQ = 2*ceil(PQ/2);
elseif nargin ==2
      m= max(AB); 
      
  P = 2^nextpow2(2*m);
  PQ = [P, P];
elseif nargin == 3
      m=max([AB CD]); 
      P = 2^nextpow2(2*m);
      PQ = [P, P];
else
      error('Wrong number of inputs')
end