Questions about example sentences with, and the definition and usage of "Ookii"

A: × Ookii and Ookina doesn’t have any differences at all.
✓ Ookii and Ookina mean the same thing.

× You can use them both of them right the same way.
✓ But, you cannot use them both the same way.

× I use Ookina a lot as with Ookii.

× However, there is one big difference between them.
✓ There is one big difference between them.

× You can’t say ‘Kono fune ha Ookina.
✓ You can’t say ‘Kono fune wa (は) Ookina.

In my elementary Japanese class we called na な a modifier to use adjectives before the noun instead of after.