How can I change the overall width of cells in a chart?

Please try injecting this CSS codes into your jotform http://form.jotform.ca/form/51315441415243

.form-matrix-table {

  max-width: 600px;

}

.form-matrix-row-headers, .form-matrix-column-headers {

  width: 400px;

}

Let us know if this did not work.

Thanks.