"My question is how big is this buffer?"
That varies between switch devices. Generally, switches with more buffer capacity tend to be more expensive. (Much like PCs or smart phone with additional memory.)
"Plus, what would happen if received data from PC A and PC B are bigger than this buffer size? -> I am guessing that the switch would simply drop the frames and clear the buffer."
Yes, frame/packet is dropped; usually not as "buffer"likely still contains earlier received frames/packets.
" If clearing buffer keeps happening (Packet Loss) because received data is bigger than the buffer size, how does the control flow happen?"
That depends on source to destination flow management protocol. Sometimes loss goes unnoticed. When detected, loss may cause retransmission or lost may cause source to stop sending.
"Lastly, is large buffer == Queue?"
Yes and no. First "large" isn't needed, i.e. does buffer == queue? Second, queue is often used in the context of multiple frames/packets waiting, e.g. an egress queue. Although buffer can be used in that context too, it's often more often used for single frame/packet waiting area (e.g. a memory area to save a frame/packets actually being actively received from the wire) or waiting area perhaps more dynamically managed, e.g. memory buffers allocated for multiple received frames/packets that haven't be specifically processed for a particular destination egress queue yet. (The last, though, might also be called a queue.) Again, there's overlap in the usage of the two terms, including either might be used.
https://en.wikipedia.org/wiki/Data_bufferhttps://en.wikipedia.org/wiki/Queue_(abstract_data_type)
