I use Opera with xmonad 0.9 and I have never noticed a problem with dialog sizes. I don’t use a password, but the download dialogs and all other dialogs are appropriately sized. What version of Opera are you running? I’m running the latest under Arch Linux, but I didn’t have any trouble under Ubuntu several years ago either. What distro are you using? Can you resize the dialog boxes? If you do so, do new dialogs open with the new size or still with the incorrect size? Is this an intermittent problem, i.e., do some download dialogs open with the correct size and some are too large? Do the same dialogs always have the same incorrect sizes or do they vary? When dialog boxes in other applications open in a size I don’t like, I often press Mod+T to tile the dialog box. (I’ve considered modifying my config to automatically tile all dialog boxes, like ion3 used to.) If you can’t fix it any other way, tiling missized dialog boxes is an easy workaround.