I used to do both cover and main illustrations in original size. But the picture book I've made ended up to be illustrated smaller then the printed version of it. This was due to the publisher who liked much more of my thumbnails then my finished illustrations (!), so that I was using a process of increasing size of small drawings on the computer, printing them on proper paper and then colorize them. I've never heard of anyone illustrating a book like this before :face_with_stuck-out_tongue_winking_eye: Wasn't my choice either. The paper had to fit my inkjet printer, but I had a very good scanner so the increasment of size was no problem. (Here's the cover.)
So, basically it depends on your equipment if you can increase the size afterwards. Some scanners just interpolate the image if you go over a certain value of dpi. You could try by just scanning any of your images with 900dpi, and print it three times the size of the original to see if you remain the quality.