It's as big as you want it to be :) Some people build 1000+ page versions, which admittedly can push the filesize up a fair bit, but for the default config it weighs in under 3MB.

How the annotations are stored depends on the device you use. I just exported my 2022 planner in the reMarkable app with annotations (been using this file since May) and it's 32MB. So not exactly slim, but not massive either.