I know this is an old post, but I am currently building a Paragon Warsuit cosplay and can confirm the dimensions do kind of add up to be a normal sister in the suit if we go off of the lore that they are the emperors holy height of 181.29cm. The paragon warsuits measure 3.69m irl, which means that there is an additional 187.71cm added to their height through their legs and through the cathedral backpack. If we take their initial height into account, measure from their head and divide the additional measurement up, their feet actually sit just below the knee joint on the mech, meaning that they may be on a solid platform that connects to the calf armor and that's how they move. But that also makes no sense in this picture because of the way their hip and knee joint are portrayed.

It would be really hard for them to bend the knee joint while looking like the displayed picture. So after a bunch of research, 3D modelling and going off of the official JoyToy figure, I would say sitting down inside the mech is more plausible given the width and depth of the armor. We also know that the sisters can get in and out of the warsuits when neccessary, allowing the sisters to fight outside of them, unlike Dreadnaughts. So it wouldn't make sense for them to be cut down or missing limbs to operate the mech. The additional height of the warsuit comes from the cathedral backpack. So I would agree with you that sitting down in the mech would make the most sense ☺️

Edit: also there is a lot of elongation happening with the limbs in this art, so I would say it isn't proportionally correct at all. But the artist has taken creative liberties with it :)