Comment 17 for bug 893644

@Luis Mascote, do you remember what you did to grow Onboard larger than the screen? I'm trying, but can't seem to figure it out. AFAIK it can happen when changing screen resolution/connected monitors, but I'm not sure how to do it in regular usage. Is this on Compiz/Unity?

Concerning the resize protection, all it does is to disable resize handles of the keyboard window and floating icon. This is to prevent accidental resizing. It wasn't actually meant as a hard lock on a specific window size. While it's active you can still use Onboard's touch handles for example (long press on move button).

There's no convenient way to reset size/position (should there be one?), but you can run:
gsettings reset-recursively org.onboard.window.landscape
optionally:
gsettings reset-recursively org.onboard.window.portrait
gsettings reset-recursively org.onboard.icon-palette.landscape
gsettings reset-recursively org.onboard.icon-palette.portrait
or look up these paths and change them in dconf-editor as Francesco suggested.