Well perosnally I have 19 regions on OSGrid hosted on 2 VPS servers, Using the sizes 200m and over is good in that it not only allows me to build as I wish, but it's also good when it comes to linking things across region boundries, and to date have several regions with tunnels that go into next door regions, having the long length helps with the border crossings.
I can understand your concerns regarding Griefers using the max prim size to do things, but I would honestly think if they wanted to do something like that they could just as well do it with the 128m limit as well..