In case of very big tanks recursive searching and filling creates lags.
https://dl.dropboxusercontent.com/u/25172450/bugs/g4fyyt.jpg

Yup, they do. Recommendation is to not create really big tanks.

So limit their sizes. It is possible to lag out whole server just by setting up a tank 15x15x15.

Nope. If you do not want your users using tanks you can set the block id to 0 to disable them.

What about a config option defaulting to unlimited size?

No.

Limiting tank sizes is a silly notion. Not only is it technically an absolute nightmare due to them being free-form tanks, it's also pointless. If a user is determined to make larger tanks they'll just build multiple large tanks.

my recommendation is that if you cant trust your users, disable the tanks. There's plenty of other tank options, nobody is forced to use openblocks ones.