#includeEigen::Matrix matrix;
#includeIn this example, we define a matrix `matrix` with `float` entries and size `mxn` that can be determined at runtime. In conclusion, Eigen is a powerful library for linear algebra in C++, and understanding the size of the matrix is essential to use the library effectively. To define the size of the Eigen matrix, we use the template parameters `Rows` and `Cols`, and for dynamic size, we use the `MatrixXf` class.int m = 3; int n = 4; Eigen::MatrixXf matrix(m, n);