Reducing the size of mysql files
u can do something like this:
add this to /etc/my.cnf

[mysqld]
expire_logs_days=3

and mysqld will delete them logs for you



if you talking about reduce the size of the table , i would advise this:

1st u need to sort from workbench or phpmyadmin , which tables consumes the most.
2nd maybe u can start plan on how to reduce that 'biggest table'
3rd u think to think or consult with your superior , how long u need to keep the record , some company might wanna keep 3months record only ,then dump and keep somewhere(cloud or Harddisk) , or 1 year or 3 years . 
4th before dump , maybe u can create archive table. 
eg: your original table 'test' , archive table rename as 'test_archive' then move older records or records that u want to dump there.
5th dump and move to somewhere safer.