Please post the complete error message, such that we do not have to guess the line, which causes the problem. I guess boldly:
C(g+1,:) = y;
Here y is a [1, q+1] row vector. Assigning it to a row of C requires, the C has the same number of columns. But this cannot be true over multiple iterations.
You did not include comments in the code. Without a description of what you want to achieve, suggestion a solution is an educated guess only.
[EDITED] The number of peaks differ between the iterations (I guess), then store them in a cell array:
Av = 0:pi/2:4*pi;
D = cell(1, numel(Av));
for A = Av
...
[pks,locs] = findpeaks(C(g+1,:));
D{g+1} = pks;
...
