GOING BIG
Working with such large paper models requires support. 3D printing at this scale has so much imperfection, time and risk of failure, it would be very heavy and require a complex web of internal support. As an art form, sculpting/mold pouring also isn't under threat from this tech in my eyes. For my pieces, shoddy woodworking as a skeleton does the job and I pad the areas were wood contacts the paper with pool noodles, pillow filling wrapped in fabric, fire hazard stuff. 120 lb paper is flexible, surprisingly resilient, and the final product is so lightweight it's never going to warp if properly supported. I use wood glue so the paper will rip before the tabs will ever become unsecured.
I've had to finish assembly after transport on site, that's something I avoid, logistic crap. I tend to base the size of my largest pieces around standard dimensions. Standard height of a ceiling, standard width and height of a door and double doors. LxWxH is not a determine factor, you can maneuver a piece diagonally and twist it around to make it fit through a frame.
MODELING
idk. I have no formal training. I studied art from my favorite games, Bloodborne and Dark Souls, looked at art, texture-less models. At first I only made symmetrical models and added asymmetrical parts afterwards. Just get blender go on youtube and practice. I copied a bunch of low poly tutorials and then my first model was a huge leap into complexity. If you're making a cat and the proportions look weird just look at a reference and keep messing with it till it seems right. Once you have the proportions right you can make the polygons look more geometric-y. I am not a creative person. I do this in reverence of objective beauty and other's art, try to reverently replicate it in this form. Thanks for reading I hope this helped or inspired you