As you can see here 14 year old Olly Bridge is using the 2011 69cm wide North Race, so it seems there is no racing advantage in going smaller than 70 cm other than maybe cost.
The newest 70 cm boards are reportedly easier to ride than previous models, so learning on a smaller race board would be more difficult but you then have a higher skill level and switching to the 70 cm board shouldn't be difficult.
Sheeting with one hand while steering with the other means you can keep the ideal stance without having to bend to sheet out, so you can hold more power and not unbalance the board, so you could adapt your bar for that and that would be a large part of the skill you would need. You can See Adam Koch doing that in this video.
You need to be able to make all those adjustments without thinking so you can concentrate on what your opponents are doing and check the wind angles and buoy positions for deciding when to tack.