how much of a Newton would you flick a marble with?!

It depends on the data:

- how big is the marble?
- how fast do you want to shoot the marble?

Take the example of a marble of 1 cm in diameter (3/8 of an inch), or 0.005m in radius, to be shot at a speed of 1m/s from rest within 0.1 sec.

Volume
= 4πr³/3
= 5.24*10^(-7)
With a density of (glass) of approximately
2500 kg/m²

mass, m
=2500*5.24*10(-7)
=1.31*10^(-3)

Acceleration, a, is from rest to 1m/s in 0.1 sec.
= 1m/s /0.1 s
=10 m/s²

Force (in Newton) required
= ma
=1.31*10^(-3) * 10 N
= 0.013 N