There are a few ways. You can do it manually with the inspector and quickstamp or rt. mouse click and select get info. You can then adjust the height and width of a note.
The other way is to use a Prototype. You can create a prototype and then assign that to your notes. Your notes will the inherit the width and height, color, etc. from the prototype.
Hve you seen my videos: Mastering Tinderbox: Training Videos (Complete List)?
Take a look at this one: Tinderbox Training Video 04 - Getting started with prototypes.