Set Default Finder Window Size In Mac OS X
Step 1: Launch a Finder window and prepare to be amazed at the random size that it chooses. Using the normal methods, set the Finder window to your own preferred size.
Step 2: Close all open windows by holding down CMD+Option+W and clicking the Close button in the top-left corner.
Step 3: Now, hold down the Option key on the Mac keyboard and right-click on the Finder icon.
Step 4: Select the Relaunch option from the contextual menu that appears.