The tools section at the top of the page shows a Feather. adjustment, however, this must be set before the lasso is used, and does not work once a selection is made.
To alter the feather of a mask once it is made, right click inside the selection and select the Feather feature and enter a radius there.
Go into Quickmask Mode (Shortcut is Q) to visualise the effect of the feather.
Clicking on Select and Mask in the Lasso Toolbar can also be used to refine the mask.
