You will receive differing opinions and some will say that if you do not use the 'paper towel method' to germinate you did it wrong, or at least not the best way. They will not necessarily be correct when that is said.
You will tend to find a handful of older growers who have tried various different ways to pop beans will say germing in the soil will result in more vigorous healthier plants. I am one who says just that based on my experience.
BUT ... it can be a bitch at times. Soil with good drainage can be difficult to keep moist enough at the surface for beans to pop. Because of this many who tried germing in soil had low germ rates and decided it was not the way to go. If someone has good drainage, like they should, and they want to germ in soil what they need to do is to fill their pots and then scoop out a small amount of soil in the center of their pots, or just not totally fill them to begin with, and fill it with a soil mix that will retain more moisture .... that or drip a bit of water on the beans fairly often so the soil remains moist.
As to your question about pot size and germing. Well .... is there a 'pot' any larger than the planet earth being used by anyone right now? Beans will pop in the ground and it would be pretty difficult to bring together a larger mass of soil to germ in that that ... right?
The size of a pot will not in any way effect beans popping as long as the beans can remain moist enough.
When it comes to pot sizes in general many people grow in pots that are to small and they tend to begin plants in small pots, or plastic cups, and repot once or twice or more times. I do not believe in doing that. I never use anything smaller than a 5-gallon pot, though 4-gallon pots might have been OK for some strains I have grown. Many times I use 7-gallon pots and when I was growing outside on my deck I used pots that were 15-gallons and larger.
When you start plants in small pots/containers you risk rootbound plants and the various problems that goes with the rootbound condition. You also risk damaging plants when you re-pot them, either through handling breakage and or shock. Part of becoming a better grower is learning to avoid problems and not just how to deal with them when they occur so I really do not see any reason to risk problems that can easily be avoided.
