I bet that the Heemskerk Lunet 10 does make a good chance as the smallest 'national park like' park, as it is a rather small bit of ground owned by Noordhollands Landschap.
Here is a link to it on google maps.
And here is a link to the charity that manages it as well as several others in the area.
I can not find any official information, the charity does not give it on their site. Google maps gives me about 100 x 82 meter, which would give me 8200 m², but that is not very precise.