Southern pine is the strongest dimension lumber. Oak is denser, but southern pine is stronger.
Do you want me to size southern pine or oak?
Is it supporting one flop only or an attic, too? What is the joist length on each side? Will it support any ceramic tile!
One floor only...
I'm using northern red oak grade no. 1
You need two 4x14
No problem. I'm happy to help. Good luck with your project