Yes, the “period length” is the shard equivalent of the main chain “block time”.
The period length is a parameter which can be specified in absolute terms (e.g. 5 seconds, as in this scheme) or specified as a multiple of block times (e.g. 5 block times, as in the first version of this scheme). The number of periods in 20 minutes is dictated by the choice of parameter. My preference would be short low-variance periods, e.g. 5-second periods.