In Yosemite, I tested one suggestion after I was unable to set a
window’s default opening behavior. I created a model window,
closed it, and then closed all windows (Command-Option-W, Option
plus close button, or hold down Option while selecting File and
choose Close All Windows). Then I held down Option and
right-clicked the Finder icon in the Dock and selected Relaunch.
Voila! After that point, whenever I resized a Finder window, all
subsequent windows would open in the same dimensions, offset
slightly in location, including the width of the sidebar. I tried
this in the current public El Capitan beta, but had zero luck.
Perhaps it will be fixed by release.