Anyway the reason I was trying with \renewcommand was because I found out that both font size of the quote and the vertical space between the quote and the main text are regulated by changing \renewcommand, so I assumed it can be done with the width too. Now I know better.
Just for the record, if anybody ever bumps to this thread the code for the font size change is:
What you found out about macros needs going a little more into details concerning environments. You can consider any environment as a group that is introduced by an \envname and ended by an \endenvname command. You can see that in the declaration of the quote environment above where you can find a \list and \endlist command which represents nothing more than a list environment.