Each note in the Note Pad has a maximum size of 4000 bytes. To check the current note size, tap on the little circled "i" icon at the top left of the note. If you are working with raw text in the default 18 point text size, you can write up to 12 screens of text per note.
Another limitation is that you can have approximately 50 items in a note, where an item can be a shape, ink (unconverted text), or a paragraph of converted text.
Shapes and ink are much more memory intensive objects than straight text.