I will be completing my 400 gallon pond in a couple of weeks and want to build a trickle tower.

How big or how many bio-balls would I need to gas-off ammonia? As I understand it you can trickle to bio-filtrate the ammonia, or simply "gas-it-off". Nitrates can also be gassed-off, correct? But with 400 gallons indoors, how huge of a container or two do I need?

Oops. Also, I've heard of a "continuous drip" system now. It seems more complicated, but if anybody knows about it, I would love to hear, or see a link.

You guys are awesome.

A properly designed TT will de-gas SOME ammonia. It all depends on your water parameters on how much can be potentially de-gassed. Ph playing a big factor.
There are so many different ways too make trickles that your options will be endless. E.G. Do you want it over the pond? Do you want it hidden?
For 400 gallons if you can provide 1 cubic foot of media with 400 gph of flow you will have a superior bio/degassing filter. Just make sure you provide plenty of mechanical filtration FIRST and use a media that is going too last and IMO provide the media a dark place too grow. Some like too grow algae on theirs but that defeats the purpose.
Some love lava rock but I think it sucks since eventially you will need too replace it. Look at more of the plastic media available and if you can provide it CLEAN water it will only get better and better with absolutely no maintenance or replacement costs.
Check out www.aquaticeco.com. They have everything media wise.