Though they'd definitely need to overhaul research to accommodate this. Currently size = better for most part, even with scaling research costs. Any OPM in the current meta is going to be obsoleted very quickly.
If I were to overhaul size, I'd overhaul trade as a whole. Give planets adaptive (or adjustable if you're authoritarian) living conditions, make it so trade also distributes supply towards other planets, and give bonuses or reduce size by adding infrastructure.
Currently, high stability is really easy to obtain. Like a HOI4 focus path, to solve stability, just press the "living conditions" button and keep the happy number up. Having a massive empire means some sectors will typically have varying degrees of livelihoods, and it wasn't simply worth it to develop infrastructure there. Even the USSR couldn't produce enough equity to even give the poorest sectors decent conditions.
Trade, as it currently stands, is a just an electrician who lives in a cubicle. They wave their hands magically and then produce energy/unity/goods. Instead, trade should instead act like a supply line. Clerks allow for the efficient distribution of goods across interstellar lines. The more clerks you have, the greater supply you can give to other planets nearby. Just like how Strait of Malacca was the trade hub between Europe/Middle-east and Asia, a trade planet between an Ecumenopolis and an industrial planet allows for the goods to get to one side to the other. If it's far away, use a gateway or hyperrelay to extend the range. Everyone is usually happier from trade, but also more productive.
High trade gives the planet high stability, but also supports research and government, and everyone else as a whole. Colonies are awful places to live at, lacking in basic goods, but in Stellaris, you suddenly have an entire polytechnic institute in a year. You need trade hubs to bring particle accelerators, microscopes, and other tools to move technology across the area. The same can be applied to everyone. Have a trade hub, and you supply equipment to improve output.
Nearby neighbors would also passively trade with your industrial/trade worlds, and they could sustain your colonies nearby. Having a trade agreement could become a double-edged sword. You can profit much more, but you would share your industrial capacity with a fellow empire. Megacorps can abuse this fact, and harvest industrial capacity, or distribute industrial capacity to other planets. You can play like an asshole and leech any bit of supply, or you can distribute to gain a fraction of any resource.
Overall, actual trade would make "big size = typically bad" by simulating empire sprawl, leading to rebellions, and produce a more organic experience.