Japanese Onions

Hi, I'm very new to gardening so forgive any amateurish questions.  

I would like to grow Japanese onions.  They say to sow them in mid August straight into the prepared ground.

My dilemma is I am renting and so shouldn't really dig up the garden.  I was hoping to grow them in pots or poly pots, but as I have no idea as to how big they will get or how much soil they will need I'm not sure if this is the way to go.  Also how many onions would I get from each plant, would I need many large pots in the garden?  

I was planning on sowing the seeds in little pots and then transplanting to the larger pots, is this okay to do?  And could I start now if I start them off in little pots in my plastic greenhouse?

Thanks for any help in advance

they are pretty big,too big for pots i'd think ,weve just picked ours and they were grapefruit sized.

Yes Loads .
Carrots -choose short stumpy quick growing ( summer) type.( Harvest before frosts)
Salad-cut and come again/ Rocket / Lambs lettuce ( need some protection outside in cooler weather)
Beetroot - thin out to reach globe size ( no good after frost)
Radish- quick growing( no good after frost)
Spinach-sow every couple of weeks ( need protection once weather cools )
Herbs -Basil ( up till frosts outside but better inside if the summer is wet and cool)
Never tried greens in pots but I think you could do some spring cabbage plants or dwarf Kale ( no protection needed except netting for butterflies in summer ?? and pigeons ! )

All these will need large pots , larger than a bucket !! and will need to be watered if we ever get any dry spells .

I'm sure others will suggest more.