So just spawn the object, get the size and move it then.
I made a “linear stairs” brush that spawns a cube, scales it to the appropriate step dimensions and moves it into place.
All meshes are created, then scaled and moved…
I also made a Fasade system that follows the same route. (See screenshots at the end of the post)
https://forums.unrealengine.com/showthread.php?67946-How-to-change-mobility-setting-for-new-static-mesh-components&highlight=kvogler
Here I scale an object to absolute dimension, regardless of its initial native size:
(The pivot needs to in the center for this to work, but my assets account for that)
In this case, create the first mesh, examine it and then run the loop from there (minus one tile).