You can find them in the book by Wilson Rawls as they exist nowhere else. As far as size, fictional plants can be any size you wish.
Big stars
A red giant.
A giant storm.
because they are very very big, and they simply are giant, not big but giants, so they got its name from their size.
All ferns are seedless
100 times diameter of the sun.
Dubhe is not a red giant but rather a subgiant star. It is the brightest star in the Big Dipper asterism and is classified as an evolved F-type subgiant star.
It will be big enough to kill us.
No, Jupiter is not a red giant. Jupiter is a gas giant planet in our solar system, primarily composed of hydrogen and helium, while red giants are much larger dying stars that have exhausted their hydrogen fuel and expanded.
In the end of the book, according to an old Indian legend, red ferns can only be planted by an angel
Jupiter has the big red spot and it is a hurricane.
A red giant will be about 10 to a 100 times bigger than our Sun. However that is just a red giant. A red supergiant will be about 100 to 1,500 times bigger than the Sun, whereas a red hypergiant will be anything greater that 1,500 times that of the Sun - Like VY Canis Majoris.