Pablo - why are you doing this row by row? Why not just resize the matrix as
A_101 = imresize(A, [10,101]);
For your code
A_101(i) = imresize(A(1,:), [i,101]);
if you really wanted to do this row by row, then you would need to adjust where the i or row number is being used in the code. Also, you need to realize that you are assigning a row (from the imresize output) to perhaps a single value in your matrix. Instead try
A_101 = zeros(10,101);
A_101(k,:) = imresize(A(k,:), [1,101]);
Note how k is used (instead of i) and how we pre-size the A_101 array, assigning a row (which is a resized kth row of A) to it on each iteration of the loop. But I would recommend against this approach unless there is some requirement to do the resizing row by row.