Auto size/center gtlf model

trying to make a GLTF/GLB loader, for example upload the model and it will be auto scaled and centered to a box set in the middle of the scene, iv pretty much got things working as i want, but im stumped , esp with my lack of knowledge of 3dmodels, as some models do not auto scale properly.

using a couple of example models from skethfab for testing here is 2 diff scenario’s (attached images)

if you see the halo model fits perfectly the way i want inside the green box (the red box is just a helper for testing which surrounds i guess t-pose? for lack of terminology , from the skinnedmesh animation) and the banana man does not scale properly (it seems to center ok)

does anyone have any way around fixing this? or is this a limitation?

*im guessing its something to do with hierarchy?, how the model was created from parent etc?.
*also notice the t-pose (red box) is rotated and diff scale to begin with on each, also im guesing by the looks of it, its maybe scaling the “t-pose” of bananaman correctly but not the skinned mesh and vice versa on halo.
*also bananaman seems to have 3 animations attached (only need to use the first which works)

p.s, i only need to to scale and fit the green box height wize and the rest just needs to be proportional.