Amount of data in a database table I am trying to run an alter query on a table, and its giving me a 

Lost connection to MySQL server during query
MySQL server has gone away.


The query that I am running is 

 alter table table_name modify column_name DOUBLE ( 16, 2) default 8.5;


I am guessing that the table contains a lot of data, and hence the alter is taking a lot of time, and the server connection is lost.

My query is, 

Firstly, 

How do I find out the amount of data present in the table ? ( in MB/GB i suppose).

Secondly, 
 In case the table does contain a huge amount of data, then how do I perform the alter on this table ?
You need to check the server's error log(s).  There are too many situations that could cause this problem.  For example:


The table is corrupt, and InnoDB detects this while reading an invalid page checksum.  The default behavior here is to shutdown to force an administrator to intervene.
Your server is over configured to use way too much memory.  The OOM killer terminates mysqld.
There is no problem at all.  The client disconnects, but the server keeps running the ALTER statement, and it eventually finishes.