How a Light Node can get a block?
Suppose that the light client wants to retrieve the complete block number 1234. I've seen on the documentation the getdata message, which other nodes can reply with a block message.
Do you have any hints on how to evaluate how many bytes the light client must send for the getdata message and how many bytes must be received?

It depends on how big the block is. The block is 0-1 MB prior to segregated witness activating, and 0-4 MB afterward. In general, the earlier the block is from, the less data it contains.
There's some overhead for the message header, but it's 24 bytes, so it doesn't really matter.