So open your Note(whatever size it is) then touch the outer part of the note with your finger until it turns blue then drag it to the outside of your screen to make it bigger, or you can use your S Pen to do the same but when you do this you will see a double arrow letting you know you can extend the box.
Basically the reason this happens is at some point you have opened a note and made it smaller then closed or saved it, then next time you open a note it will open to this same size until you maximise it yourself and close/save it again, if that all makes sense.