The quetion is now: what is an adaequate size? In your example the title needs two lines. But apart from that size seems reasonable for me for different reasons:
To determine line length for optimum readability, a good guideline is between 9 and 12 words for unjustified text. Fewer words may cause the sentence structure to break up, and may also result in too many hyphenations. Both of these reduce readability. Source
Furthermore i expect for better handling a reltively smaller size than in the main ui
As far as I can see, Bear will open in the size and position last used by extra windows.
So if you adjust the size and position to what you prefer, then next time, Bear will open new windows to that size (and position if only one extra window)