How can you change variable-sized data to fixed-sized data?

I am wishing to use multiple blocks and operation that do not support variable-sized data. The source of data is variable-sized and as such this propogates throughout the model. How do you change the data to be fixed? The size of the data remains the same throughout. The operations only change the value of the data for example through ffts, products, conjugates etc

[Scott MacKenzie]

There's not a lot to go on in your question. You might consider providing more details on the type of data (vectors, matrices, numeric, char, ...), how they are organized (cells, tables, ...), and the sort of operations required.

[Joshua Ford]

I am wishing to use assignment and variable selector blocks on double-precision data of a 512x2102 matrix. I am wanting to complete an fft of each individual column of the matrix in a for iterator block. The 512x2102 is stated as a variable sized-data size. This data comes from a MATLAB function block.

In the future the data type will be changed to fixed-point hence why I am operating the fft in Simulink and not a matlab function block.