It should cover doors of any size
you can cover up to ten doors with an illusion (equivalent to the illusory object function of the minor illusion spell) to make them appear as plain sections of wall.
What the spell is saying is that a door is covered with an illusion that is equivalent to the illusion we find in minor illusion. That sets up a bit of a conflict because the rules of minor illusion don't allow it to cover any doors greater than 5ft tall. If you look at the real world, the average door size is about 6'8" x 3"1. If this restriction (and average door size) were to hold true in this case, the spell would not be able to cover almost any door.
Logically, the spell says that one of its primary functions it that it does cover doors. Thus, in order to function properly, the spell must be read as overriding the size limitation of minor illusion. It seems reasonable in this light to think that by saying "you can cover [the doors]" the spell is actually explicitly (albeit unclearly) saying that the illusion will cover the door and otherwise has the properties outlined by minor illusion. By saying this, the door-covering function of the spell makes sense and becomes useful. And, as a result of this reading, there is also no restriction on the size of doors that can be covered.
1 - There are no rules that say this is the case in the fantasy worlds of D&D. However, D&D is full of creatures that are much bigger than medium so, if anything, in many areas the door size could actually be greater than the real-world average.