Beamforming was introduced in 802.11n because MIMO technology was already available at that time.
In 802.11ac, the beamforming has been modified a bit, which can support sending beamforming feedback requests to multiple users at the same time.
It is like this.
First, the initiator (beamformer) sends a packet named NDP Announcement to the initiator (beamformee). As the name suggests, NDP is Null Data Packet. This is an empty packet, without MSDU, but only a PLCP header and FCS. NDPA is just that there will be an NDP packet next. After a SIFS, the initiator can estimate the channel information after receiving the NDP. It is hard to say what is specific.
But the size of the reply message is probably related to these things:
For example, the number of sub-carriers, such as data accuracy, is 6/10bits for single user, 12/16bits for multiuser, and the number of angles. For 2x2 it is 2, corresponding to 3x3 is 6, and corresponding to 8x8 is 56. The maximum is about 217kbits, and the minimum is 1040bits.
