Q1. How long should the challenge run for?
One month or two depending on the time of year.
Q2. Should contestants be allowed more than one entry?
Only one entry please.
Q3. Should there be restrictions on size. IE: 800 x 600 etc. ?
I like Dan's  size for this one 1024 x1024.
Q4. What would be the best time for the entry deadline? (12 noon, 12 midnight etc)
Midnight. But let us know the time zone.
Q5. What would be the best time to end voting? (12 noon, 12 midnight etc)
Same as above.
Q6. How long should voting be allowed? (2 days, 3 days,...)
3 days.
Q7. You can't post your finished entries in the galleries until the challenge is closed.
Agreed. And I know I'm guilty of that one.