Inherit Size from Simulink

The default size is -1, which means that the variable inherits its size based on the value of the Scope property:

Variables with the Scope property set to Data Store Memory always inherent their size.

Customize Variable Sizes

You can also manually set the size of variables to a scalar, vector, or matrix. To specify the size as a scalar, set Size to 1 or clear the parameter. To specify the size as a vector or a matrix, enter a row vector with positive integers in [row column] format. For example, to define a column vector of size 6, set the Size property to [6 1]. To define a row vector of size 5, set the Size property to [1 5]. To define a matrix of data size 3-by-3, set the Size property to [3 3].

Before R2021b: MATLAB Function blocks output column vectors as one-dimensional data.

MATLAB Function blocks retain their dimensionality by default. To maintain the old behavior, clear the Interpret output column vectors as one-dimensional data property in the Property Inspector or Model Explorer. You can then use a single integer in Size to define the size of an output column vector. See Interpret output column vectors as one-dimensional data. (since R2021b)

If you do not know the upper limit of the dimension size you need, specify an unbounded dimension with Inf (since R2023b). For example, if you do not know the upper bound of the columns, but you have a maximum size of two rows, set Size to [2 Inf]. To specify an unbounded dimension:

For more information, see Unbounded Variable-Size Signals and Unbounded Variable-Size Signal Basic Operations.

You can also set the Size property with an expression. The expressions can include:

These expressions must output a positive integer or two positive integers in [row column] format. Otherwise, the specified size produces an error at model compilation. For example, consider a MATLAB Function block with variables k, x, and y that all have the Scope property set to Parameter. Here, k is a positive integer and x and y are matrices. If you create new variables for the block, you can define the Size property with any of these expressions:

k+1
size(x)
min(size(y))

Custom Size Property Example

This example uses a MATLAB Function block with an input variable that adjusts its size based on the value of a base workspace variable.

function y = fcn(u)

y = 2*u;

Simulation and Size Matching

After you build the model, the CompiledSize column in the Model Explorer displays the actual size used in the simulation. If the value of the Size property is not -1 and does not match the actual size, a mismatch error occurs during model compilation.