To let Finder always open a folder with default window width and height The Finder seems to remember the width and height of window that last time I open that particular folder and set such. 
In other words, if I opened a folder and make the window really wide, the next time (after closing it) I open that folder, the window is still very wide. 
This remains true even if I restarted computer, and even if I delete the .DS_store inside the root of that folder. 

How to let Finder forget all previously-remembered window size of every folder in this computer? 
I want that every time I open a new folder, Finder always apply the default size, the setting shipped with MacBook (This is about 1/2 width and 1/2 height of the whole window). 

Though this is not big deal, it somewhat annoys me that when I have widened a window for some reason, the next time I open it, I have to manually adjust it back to smaller width and height for ease of reading, or the other way round.
This is extreme, but works: (see also Remove .DS_Store files from Mac OS X)

sudo find ~ -name ".DS_Store" -depth -exec rm {} \;


Restart and observe that newly opened finder windows are of uniform size. This deletes all ".DS_Store"'s. It then seems that they store relevant data regarding window size.

I did this partly out of curiosity, or maybe I have compulsive personality. I don't recommend normal users to mess up with your computer like this, though.