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You cannot assign variable-size matrices to fixed-size matrices in generated code. Consider this example:

function Y = example_mismatch1(n) %#codegen
assert(n < 10);
B = ones(n,n);
A = magic(3);
A(1) = mean(A(:));
if (n == 3)
    A = B;
end
Y = A;

Compiling this function produces this error:

??? Dimension 1 is fixed on the left-hand side 
but varies on the right ...

There are several ways to fix this error:

If you assign an empty matrix [] to variable-size data, MATLAB^® might silently reshape the data in generated code to match a coder.varsize specification. For example:

function Y = test(u) %#codegen
Y = [];
coder.varsize('Y', [1 10]);
if u < 0
    Y = [Y u];
end

In this example, coder.varsize defines Y as a column vector of up to 10 elements, so its first dimension is fixed at size 1. The statement Y = [] designates the first dimension of Y as 0, creating a mismatch. The right hand side of the assignment is an empty matrix and the left hand side is a variable-size vector. In this case, MATLAB reshapes the empty matrix Y = [] in generated code to Y = zeros(1,0) so it matches the coder.varsize specification.

You can define variable-size data by assigning a variable to a matrix with nonconstant dimensions. For example:

function y = dims_vary(u) %#codegen
if (u > 0)
    y = ones(3,u);
else
    y = zeros(3,1);
end

However, compiling this function generates an error because you did not specify an upper bound for u.

There are several ways to fix the problem: