Hello,
1 - The database size is not really a problem as long as your server can handle it.
2 - It's not recommended to delete old orders, because you need to keep providing the order history to your customers.
3 - You should check which tables are big in your server manager.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.