Hm, currently there is no simple way to recalculate the size to the text’s size, so either manually set it to a fitting size or hide/disable the background by either setting it to a transparent color and enable transparency or set the relief to None (note, that you won’t be able to select the item in the editor frame but only in the structure view with that relief style).
I’ll see if I can add a convenient function to resize the frame appropriately in the next release.