Suggestions for changes to annotations should go in this thread: https://forum.inaturalist.org/t/lets-talk-annotations/627/ (I do not know if there is currently any development work being done on annotations, but it nonetheless makes sense for all discussion to be collected in a single thread).
Annotations are mostly about information related to life stage and phenology; “size” is not inherently connected to this and it is also not clear to me how a numerical value could be integrated into the annotation system, which is based on a fixed set of options. An observation field might be more useful for this purpose.