How did Extranonce2_size change? From stratum,I know that the coinbase is composed of "Coinb1 +Extranonce1 + Extranonce2 + Coinb2".

How did Extranonce2_size change ? 

Will the Extranonce2_size change when I connect to a pool?
Miner sent message mining.subscribe to pool

mining.subscribe("user agent/version", "extranonce1")


Pool reply with 

[[["mining.set_difficulty", "subscription id 1"],
["mining.notify", "subscription id 2"]],
"extranonce1",
extranonce2_size]


Pool set size of extranonce2