Small is relative, but I've viewed (and occasionally copied and/or drawn) Rembrandts in maybe a dozen or so museums in the US and Europe and have never seen any close in size to the ones shown in the article -- those are much smaller than was typical for him. This Wikipedia page[1] should give you an idea of typical dimensions.
> would you expect significant deterioration of the paint if it was actually painted 400 years ago?
That depends very much on how they were cared for, and what support they are on (e.g. copper paintings tend to age very well; paintings on canvas somewhat less so); but certainly there are many paintings in pretty good shape that date from the seventeenth century.
(Not a professional appraiser, just a painter who occasionally sells his own work and a big Rembrandt fan, but I'd be curious to see a picture of your painting if you ever cared to share ...my email is on my website linked to my profile. Good luck!)
[1] https://en.wikipedia.org/wiki/List_of_paintings_by_Rembrandt