What's the maximum packet size when we are using LLPM with connection interval 1 ms?
The maximum payload for LLPM connections is 27 bytes.
Update: Note that this is not enforced, but DLE in conjunction with LLPM can lead to undefined behavior, which is why the practical limit is 27 bytes.
Do 27 bytes refer to Data length (radio packet size over air) or ATT_MTU?
ATT_MTU = Data length - 4 (4 bytes overhead on Link-layer).
This refers to the link layer payload, so essentially it means any "normal" BLE packet where you do not use data length extension (DLE).