I use the official RuneScape client and have to keep manually restoring a specific window size to get the game's interface back in order each time the client is re-opened. Is there a way to keep the same window size?

 

Thanks in advance!

Nope. The only options are to go with a 'default' window size - e.g. the size it launches, maximised or one of the 'snap to' sizes various os offer by dragging in certain ways.

Here is an app which allows you to size a window to a preset size. I'm not sure if this is exactly what you're looking for, but it may work for you.

http://www.brianapps.net/sizer/