I wonder if making a second and distinct tool for placing a secondary house (no automatic refresh) on an acct would allow for a more restricted selection of sizes.
I do believe, the second house thing should not only work for Siege but on all shards. With allowing it on all shards, players would move their house, if they choose a new shard as second shard and they would not block for new active houses on Siege.
A tools, that would only allow to place a house of max 100 tiles that must be refreshed once a month would work fine.
A player could rezise his primary house to below 100 tiles and switch, so the second house become primary and old house become second house, then he could rezise the new primary house.
I can see, if a player place a new house, and already have both a primary and a secondary house, the choose of tools, would choose what old house to decay. If using tools for primary house, old primary house would decay.
I really hope Devs will come up with something soon, that both help the homeless Siege players but also help Siege so only active houses will stay.