I just wanted to add another report of this issue. My Dell laptop has a 3200x1800 display and scaling is set to the (Dell recommended) 200%. Most programs display great, but in Opera checkboxes and radio buttons aren't properly scaled and just appear as tiny dots. As others have noted, it's almost impossible to tell if a checkbox is checked or empty.
Usually I just end up using another browser, but I'd love to see this addressed in Opera.
By way of an update, a few days back I deleted my Opera program folder and did a fresh install of v35...the scaling issues are now all resolved. There must have been some remnants from older versions in the install folder that were causing issues.
The fresh install also cleared up some HTML5 video problems I was having.