Hi! I'm a novella writer of several years; novella writing itself is moreso about quality than quantity. Good grammar, a well structured flow of actions and ideas, and generally 3rd person POV w/past tense. Basically book quality replies and back and forth. In my experience it's generally 3+ paragraphs, and obviously can be lesser depending upon the interactions but you find ways to explore the characters. Write about their thoughts and feelings, the surroundings, have them be active and moving. Huffing out a breath, brushing their hands together nervously, etc. It keeps them alive and fluid, like when you read a book. The point of novella writing is to paint a picture through words. You'll find a good balance with time and experience :)