| Well, capacitance is k*eps0*A/d. For a 30 gallon rubbermaid container we have approximately the following:
k for hdpe is 3.2 (I think that's what it's made of, but it's close enough anyway).
eps0 is 8.854*10-12.
I'm going to estimate area as a 2' diameter by 4' high cylindrical surface, which comes out to 2.6 m2.
d is about .025 m.
So we get:
3.2*8.854*10-12*2.6/.025 = ~3 nF. Yes, that's right. three nano farads. Disappointing, isn't it? That's about what I expected though. Leyden jars are just not great capacitors.
As far as the length of the spark goes, that depends primarily on the voltage (potential). The energy of the spark depends on the voltage and the capacitance (Q=C*V). So the higher capacitance you have, the brighter the spark will be. You can charge a Leyden jar to a fairly high voltage I believe, but it just doesn't store much energy. | 
