SSRS - Multi-Select Drop Down parameter cannot be resized

This issue happens with IE10

To get the resize triangle to appear on the drop-down list

• Press F12 to open the Developer Tab in IE10 (Or Click on the Gear> F12 Developer Tools)
• The developer tab will open on the bottom
• Click "Emulation" in the Far right on the developer tab
• Change Document Mode to 9
• The triangle should work correctly now
• You cannot save this configuration

For a permanent solution,
Download this and use IE Enterprise mode to force the compatibility view of each website

https://technet.microsoft.com/itpro/internet-explorer/ie11-deploy-guide/what-is-enterprise-mode