Hello @Kenneth1,
On the Grid's on initialize event, you can call the following snippet of JS to achieve this. This will make the first column (index 0) have auto fit.
GridAPI.GridManager.GetGridById($parameters.GridWidgetId).provider.autoSizeColumn(0);
If you want all columns to auto fit, you can call:
GridAPI.GridManager.GetGridById($parameters.GridWidgetId).provider.autoSizeColumns();
I've attached a sample where you can see it working. We'll also analyze this internally and see if it makes sense for us to create a client action for this.