In Portal 2.0, when you define the fields, you can specify a
sizeHintvalue. A value of 1 is normal size, 0.5 will give you 1/2 width. You can play around with those values to ensure that each column is appropriately sized for the data you expect in that column.
Here’s a link to the documentation for this feature:
In Portal 3.0 and Workspaces, column widths cannot be changed at this time.