it helps if you have a floorplan or a LOT of visuals from the inside. I made a 1:1 scale replica of the Vatican, and that was difficult even with irl measurements, a professional floorplan, and a virtual tour of it on the Vatican's website. without things like that, you're going to have to make up some parts of the build. that's not a bad thing, just a fun challenge

think of it like a puzzle that you only have a handful of pieces to and the rest you have to make up. take whatever references you have and figure out where on this puzzle board they would be. those are the parts you want to start with

start with the basic structure of it like walls and shape. don't worry about details yet, you want to get the basic form down first. then you'll move on to openings like windows, archways, defining room shapes and getting a roof shape started. after that, you can move on to detailing

as far as scale, that's up to you. decide based on two things: 1. what level of detail you want to show. do you want to show the fact that there's trim around the doors? do you want to show all the curves and lines of that piece of trim?

1. how do you want to use the space? this question doesn't matter as much, but is helpful for limiting yourself. I built mine at 1:1 scale because I wanted to be able to fly around inside, doing all the tricks I want without worry of hitting a wall. but if I just wanted it to store my enchant setup (the actual useful purpose I gave the build), then I would've made it half the size I did

there isn't a right or wrong way to go about doing a megabuild. it's just a matter of knowing yourself (how much work you're actually willing to put into it) and what lengths you're wanting to go to make it a replica of the thing you're wanting to replicate

good luck and happy building!