Questions regarding statue farms.

I got into Terraria a little while ago, played for a bit, then came back again a few days ago. I recently started hardmode for the first time and so far things are going alright.

However I am finding myself in need of money for reforging and some other cool things (I neglected to save up much money before starting hardmode. I decided to try WoF on a lark to see how badly I would get stomped, ended up beating it first time out. Oops. xD ). I've seen a number of people on youtube and forums posting about the greatness of statue farms for money and loots, and I made a very basic shark one that got me the fins for the megashark, it's no where near as pretty or smooth running as what I've seen.

The biggest issue I'm running into is I can't seem to place the statue in the lava to instantly kill the shark as it spawns. The best I've managed so far is to place it on a platform above the lava and throw some spider minions at it. If I could post a picture of it I would, but at the moment I can't.

I've got a few questions. One, how many block long do I need to make the bass to get the thinnest bit of lava that will kill enemies, but not destroy items? I've found differing answers to this, ranging from 6-10 blocks. Two, how do I place the statue in the lava? I'm sure I'm missing something simple, but so far, the game has not let me place my shark statue in any depth of lava. And none of the "guides" I've found so far show the actual creation of the farm, just the final result, which isn't very helpful in actually building the farm.

Any insight you kind folks might be able to provide is appreciated.