I would always suggest using a css class to set cell properties as it is easier to manage across different table inserts. If you set default cell width and height values, these will be applied to all tables.

So for example, add a css rule to your template stylesheet, using a specific css class name:

.table_cell_widths td {
    width: 50%;
}

You could also create rules for specific table cells, eg: first and last:

.table_cell_widths td:first-child {
    width: 30%;
}
.table_cell_widths td:last-child {
    width: 40%;
}

Then when you insert the table, you can select "table_cell_widths" from the Classes field in the Advanced table of the Table dialog, or set this as the default Class value in the Table parameters in Editor Profiles -> Plugin Parameters -> Tables