Or How to Enjoy the Benefits of Building to Scale Without Making it Impossible for Others to Explore Your Creation!
Before we get into the solutions, first remember that you can only resize a building that is either modifiable or includes a resize script (most no-mod houses do not, at the time of this writing, include a resize script). So your safest bet is to avoid buying no-mod.
If you build mesh houses/buildings yourself I'll say right away that the best solution is to leave the wall over the door phantom. Bam! Done! You've made a building that is more accessible to all sorts of SL users and a bit easier to rescale to better suite for your customers purposes.
For those of you who have purchased your buildings and need solutions to the small door problem, read on!
The Physics Solution
Here is a mesh house I've shrunk down to about 1=1 scale. For reference, my avatar is 6'1" and the door frame is about a foot and a half or two feet taller.
So how is an 8-9' tall avatar going to fit inside that? EASILY!
First you need to find out how the physics for your structure are done. Many mesh buildings use a shell of some sort, separate from the mesh model. Not all structures do this.
If your building has no separate physics box, a quick solution is to see if the wall over the door is a separate mesh item you can simply set phantom. Even if it's connected to the wall on either side of the door you can set it all to phantom and put two invisible prims in its place.
If the building is a single structure that can't be separated you may have to set the whole thing phantom and build the physics with prims, I'll explain that a bit more later.
First you need to find out how the physics for your structure are done. Many mesh buildings use a shell of some sort, separate from the mesh model. Not all structures do this.
If your building has no separate physics box, a quick solution is to see if the wall over the door is a separate mesh item you can simply set phantom. Even if it's connected to the wall on either side of the door you can set it all to phantom and put two invisible prims in its place.
If the building is a single structure that can't be separated you may have to set the whole thing phantom and build the physics with prims, I'll explain that a bit more later.
In the meantime, let's reveal the physics prims for this house!
The prim physics boxes are blue! Notice how there's no prim covering the wall above the door! That wall is completely phantom so avatars of any size can fit. All of the interior doors for the house are the same! Leave that space blank and you have doors anyone can squeeze through.
So what if your structure doesn't use prims for the physics? Set the building phantom and build a physics shell out of prims like I did for this house! If you also set the house itself to "no-physics" you can often reduce it's land impact considerably! The root object of a linkset must have physics so you may have to link the house to a prim first, set the whole structure to "no-physics" then set the prim itself to "convex hull" for minimum land impact.
Remember to set the physics prims to "convex hull" to reduce their own land impact. After you've done all this you can often link the no-physics house to the convex hull prim shell for even more of a land impact reduction. The entire house in this example is 21 land impact.
Teleporter Solution 1
Another solution to the small doors problem is to place a teleporter above the door! Just rez a prim above the door frame, where the physics for the wall over the door begins, like so.
And one for the opposite side of the door as well.
Then you simply make the prims invisible and put a teleporter script inside each.
If you want this to be completely seamless you'll need an Experience Key. This way when an avatar collides with the prim they will automatically be teleported to the other side of the door frame. They may not even notice the teleport.
If you do not have access to an Experience, you can put a script inside the prim which informs the avatar colliding that they need to click above the door to "duck". Setting the prim to "sit when touched" and an old fashioned sit teleporter script will then be enough to achieve the effect.
Success! A door anyone can pass through!
Teleporter Solution 2
There is an alternate way to use teleporters to create doors anyone can pass through. That is to make the entire doorway itself a teleporter!
I like to put building interiors into skyboxes (reduces lag, increases framerates, and gives me more freedom in building) and the entrances/exits for these skyboxes are teleporters disguised as doors.
You can script your own or use a an existing door teleporter like Curio Obscura's "Anywhere Door".
The "Anywhere Door" is a sit teleporter. You click the door to open, then click the open doorway to be teleported to the door's destination.
My own sims now use an Experience based teleporter for a more seamless experience. You click on the door to open it and then simply walk into the open doorway to be teleported.
And there you have it! Three solutions to the problem of doors being too small for giant sized avatars! I hope these examples help you out in your own sim building adventures, if you have any questions, leave them in the comments below!