Are you referring to their width or height? If their width, do the following:
1. Open a new Finder window.
2. Resize the first column while holding down the option key to resize every column.
3. Close the window.
This should set the default properly. If it doesn't, open Home/Library/Preferences/com.apple.finder.plist in a text editor. Do a search for 'ColumnWidth'. After that should be <integer>[current width]</integer>. Change the number to the desired width and save. Then force quit (relaunch) Finder and open and close a window. Open a window again and it should be fixed.