How do i use dynamic variables with a matlab function in stateflow?

In my stateflow model I try to extend a matrix with a matlab function. I have the problem that I always get the error message: 'Computed maximum size of the output of function 'colon' is not bounded. Static memory allocation requires all sizes to be bounded. ...' Then I found that I have to define the maximum size of the matrix but it still won't work. Here is the code of my function:

function MatrixOut = ExtendMatrix(MatrixIn, Stepsize, GRVal1, GRVal2, Minutes)
coder.varsize('Matrix_Out', [30000000 4]); 
n = Minutes*60/0.02;
coder.varsize('t_vec', [1 n]);
coder.varsize('MatrixRoh', [n 2]);
Start = MatrixIn(end,1);
t_vec = Start+Stepsize:Stepsize:Start+Minutes*60;

MatrixRoh = [GRVal1.*ones(Minutes*60/StepSize,1)...
    GRVal2*ones(Minutes*60/StepSize, 1)];

MatrixOut = [MatrixIn;[t_vec' MatrixRoh sum(MatrixRoh,2)]];

end

MatrixIn is [n 4] an all are double. t_vec is temporary and double. Can somebody help please? Paul