Have always used 30cm planters myself, gives plenty of room for the roots! just remember to weigh the pot down/secure it. After all we don't want the fine 'erb crashing to the ground and being damaged in high winds! Have fun!
20cm by 16 cm roughly calculates to 1 US gallon. I would not use these size pots for the duration of the growth. I have used those before and these are the troubles I have found. Not big enough to be sturdy to support a mature plant. Not sufficient storage capacity to hold water, i.e. drys up faster than bigger pots and requires more frequent watering. Less soil requires more frequent fertilizing. Salt build-up intensifies and burning from overfertilizing may occur. Now that's not to say they won't work. If it's all you got, its all you got. So stick with them. But if you have an oppotrtunity to upgrade in size, go for it.
well if they can grow to a decent size it shuold b ok, im growing indoors and can hav the pots supported so they dont topple or anything