You don't really need the on start event to create a hash table to begin with. Get rid of it in favor of the -m switch being used.

The timer suggestion is a waste of resource. A hash table won't get lost so long as you don't close mIRC, let alone the trigger upon a nick change in this snippet. The essential one you'll need is the exit event.

There's no difference between $site and $wildsite. They are releatively the same; one is shorter and the other longer.

Hash table virtually has no size limit according to the RAM of your computer, which is what mIRC's hash table function adapts to ultimately. The more memory you have, the better for a hash table to work fast and effectively.