I asked a similar question about adjusting column widths in the envionment pane on RStudio Support, because this has been bothering me since I upgraded to the new version too.
Here was the official response:
"Unfortunately this is currently not possible but its something we're looking into implementing going forward. In the meantime, hovering over the variable names in the Environment pane should bring up a tooltip that states the full name of the variable - hopefully that will be helpful to you for this purpose."