Hi! Not sure how useful this is, but I am only using a staticmesh actor reference in the blueprint, not an actor, and not a component and using the UStaticMesh->CalculateExtendedBounds() and then UStaticMesh->GetBoundingBox(), then I am spawning the mesh component along a spline, so I also can not spawn measure and readjust as my size determines my loop count.
This works for me, no need to spawn and measure but you do need a reference to the static mesh itself somewhere in the blueprint via UStaticMesh.
Side note: SplineMeshClass->GetBounds().BoxExtent for this particular mesh was exactly half the scale of the bounding box, and bounding box also matches the Approx Size values I see on screen when opening the mesh.
Good luck!