There seems to be  a bug in Yosemite Finder.

The user settings of sidebar width etc do not seem to be persistent when you close the Finder and reopening, like we are used to in the past.

Frustrating yes, but don't feel alone.

Apple Feedback http://www.apple.com/feedback/

Seconded - I'm getting the same frustrating behavior, in that no matter what I do to resize the Finder sidebar (hold option or not, quit Finder or not, reboot or not), the change doesn't persist.  This was working before Yosemite, so something seems to have changed - if there's either a fix or a workaround I'd be very interested.

This is a bug in Yosemite since the very first beta build (10.10). Heaven knows when Apple will fix this.

Yeah, the behavior's pretty bizarre now that I mess with it - if I manually edit FK_SidebarWidth in com.apple.finder.plist and restart Finder, the _first_ window opened has the correct width.  But every subsequent window resets to default, and the property never seems to get modified except by my manual edit.  It bugs me enough that I'm willing to try one of the AppleScript workarounds, but that's pretty ugly...

Well, happen to me in 10.10.2, this bug does not appear in 10.10.1

BTW, the Sidebar size only changing it's size when open new tab if USB Device(s) connected,

everything revert back when all USB Device(s) released.

What bugging me, I cannot resize the new tab exactly like my other tab when USB Device(s) connected.

Try opening Script Editor and

Running this and change the number to whatever size you want.  You can always save it as an app if it doesn't persist, hopefully it only has to be run once though.

tell application "Finder"

  set sidebar width of every window to 280

end tell

While the next version doesn’t fix this bug I propose an different solution. But I like it so much that I will use it forever:

1. With Automator create (or copy and past from link bellow) an applescript to open a new tab in finder (from the last opened folder):

https://gist.github.com/n8henrie/0ceef75964bd153f910d

2. Before the last line of AppleScript ("end run”) copy and paste this line to set the sidebar width:

tell application "Finder" to set the sidebar width of every Finder window to 180

3. Run and test the script and set the sidebar witdh (180) to the best value for you.

4. Use OSX AppleScript Editor to Export this script to an “Add New Tab.app”. A good tip is export and save it to the folder that you want to be the default tab folder (in the case when no tab was already opened).

http://www.engadget.com/2008/01/13/applescript-saving-scripts/

5. After export and save it, move “Add New Tab.app” to Applications folder.

6. This app is going to use some Accessibility features, so is necessary add it to the list  "System Preferences-->Security and Privacy->Privacy->Accessibility” and authorize it to control the computer.

https://support.apple.com/en-us/HT202802

7. Now we need only create an keyboard shortcut (Shift+Command+T) in Finder to run “Add New Tab.app”  to open a new window (if no window already opened) or tab. After this we can use it in place of default shortcut (Command+T) to open new window or tab with correct width (see the tutorial bellow):

http://computers.tutsplus.com/tutorials/how-to-launch-any-app-with-a-keyboard-sh ortcut--mac-31463

ps: A good tip is to set this shortcut in all applications and not only in Finder to open a new finder window when necessary. You have to make some small additional steps:

8. Go to System Preferences -> Keyboard -> Application Shortcuts -> All Applications

9. Add new line with properties:

Application: All Applications

Menu Title: New Tab

Keyboard Shortcut: Shift-Command-T

10. Now, test the shortcut. This Service will appear at every app into Services menu and will work as you want.