You can set a default size, but if you've previous opened those folders in specific locations and sizes, they may remember that setting.
To set a default location and size for new Finder windows, open a brand new, fresh window (preferably without any other Finder windows open), set the position and size, then immediately close it. Any new Finder windows should then open the same way.
To restore other folder's settings to the defaults, you'd have to delete the .DS_Store file contained within it (hidden by default). There are a variety of programs out there that can help you with that, such as Onyx or TinkerTool (I believe, haven't done that in a while). They'll also remove other custom settings like backgrounds and view settings, so if you have those set on any folders, you'll have to redo them.
jW