Attribute Create SOP has this "Transform Matrix" option, so I assume it's capable to create a matrix attribute.

However my question is... how? The max size is 4. A matrix is 3x3 or 4x4. Even I type 9 or 16 in the size field, there are still 4 fields for the values. How to set its value?

raincole
However my question is... how? The max size is 4. A matrix is 3x3 or 4x4. Even I type 9 or 16 in the size field, there are still 4 fields for the values. How to set its value?

It's not possible, as you've found. An attrib wrangle or vop can set a matrix attribute value directly when needed.