I just give my daughter (grade 4) the list of her readings for the week and she chooses when she wants to read each day and which she wants to do as written narration. I think a general rule of thumb is to start with one per week, and then each year add one more per week. DD loves writing, so she started doing 1 each week last year, and is doing 2 or 3 each week this year. She varies the subjects that she does written narrations for. Some books might be easier than others, but I think it’s good to get experience with all different types of reading.
She also has the option of typing or handwriting. She does most of them typed, and gives much more detailed narrations when she does. She does like to do some in a notebook, however, so she gets the handwriting practice as well. She has a thing for notebooks!
It is normal for narrations to be shorter when written. If your daughter is concerned, you could give her the option of writing the first part and then narrating the rest orally. You could ask her to write for a specific amount of time (10 minutes, maybe?) and then continue orally, with you acting as scribe if she wants to have it all on paper.
For what it’s worth, my son in grade 2 has started volunteering to do the occasional written narration. He has done 3 so far this year. There is no way he would do it if I expected him to do it by hand, but he learned to type this summer, so he does them on the computer. He prints well and does copywork without problems, but wouldn’t be able to think about the narration if he had to concentrate on printing skills at the same time.
All the small steps add up, so I don’t mind letting them focus on one skill at a time at this stage. We’ll get to longer, more frequent, handwritten compositions as we go along.