Hold down keys when clicking the mission to customize.

Space Bar: Re-roll ships.
Control: Reset settings.

D Key: Next player faction.
F Key: Next enemy faction.
A/S Key: Decrease/Increase battle size.
E/R Key: Decrease/Increase enemy advantage factor.
I/O Key: Decrease/Increase player quality.
K/L Key: Decrease/Increase enemy quality.
B Key: Toggle fleet balancer.
T Key: Toggle time acceleration.