After some further thought I think 10M atomic iota "IOTA" human/trade decimalized units would be ideal. 1M units would be too small, enough to run into the trading issues encountered when trading in the hundreds of satoshis range. 100M units I think would be too big. 10M units gives a decimalized IOTA a 277,953,028.3277761 total supply. This is a good balance for human readability and market trading.
In chat it was brought up that markets can figure the issue out for themselves, but it will make everything much simpler if the IOTA team came up with the definition of a human unit ahead of time. In Ethereum for example, an ether is really just 1000000000000000000 wei, the atomic unit used by Ethereum's network token. So really, the supply is 79148879000000000000000000 wei, but to make things simple for people they use a decimalized unit.
I'd like to see the human decimalized unit be named simply IOTA to prevent confusion, because that's what the token has been named so far, and a computer program won't care what it's called. Essentially I'm all for making the software more efficient so I'm all for this change, but leaving it up to others to decide the units that people will deal with would be an error.