Yeah, I agree with Plane Sailing:
You could scale the effect with the size of the oil container, but say:
Improvised thrown; Dex versus Reflex, range 5/10,
* Small = One square only, 1D4 initial damage, 2 ongoing (save ends).
* Medium = Burst 1, 1D6 initial damage, 3 ongoing (save ends).
*...