what is Maximum size of a query in mysql?

the max size would be limited by the variable max_allowed_packet
so, if you do a

show variables like 'max_allowed_packet'

it will show you the limit. By default, it is set to
1047552 bytes.

If you want to increase that, add a line to the server's
my.cnf file, in the [mysqld] section :

set-variable = max_allowed_packet=2M

and restart mysql server.