If you are using the table, you should change the size of the cell
instead.
For example, lets imagine that your code was:
TextButton btn = new TextButton(...);
btn.setSize(100, 100);
table.add(btn);
You should define it like this instead:
TextButton btn = new TextButton(...);
table.add(btn).size(100, 100); // <-- resize cell, not button
Please refer this document for more information.