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