How to express size of matrix

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I am sorry if my question is not smart enough. Via this post (Is there any standard notation for specifying dimension of a matrix after the matrix symbol?), I am still bit confused with matrix notation.

Would you show me how I could express the size of square matrix called J (n x n) with n dependent on cumulative weight fraction?

$\sum_{i=1}^n \omega_i = 1$

Thank you so much for your help.