Can client size be remembered?
By client size do you mean scaling factor or window dimensions? Scaling factor is remembered across browser/launcher restarts for me, but each world has its own settings. So if you set every world to 2x scale, they should all stay that way.
Page zoom level is also remembered per-domain so if you zoom with your browser instead of using the scale buttons, it might be even easier to save scaling factor.
For window dimensions – I’m not sure. Have you tried using the launcher? Your OS might remember window dimensions for it since it’s a separate app. Release Markets link whitelist · LostCityRS/Launcher · GitHub
For the browser client, there might be a browser extension out there somewhere that is capable of resizing windows based on URL? Or you could possibly use OS tools like FancyZones to accomplish this.
Let us know if you find a good solution 