Is it just cast at 8th or 9th? Highest it can be? Maybe base level? Been wondering for a few days about this.

According to sage advice: whatever level you like - but no higher than 8th.

The reasoning is twofold:

1. The spell says "The basic use of this spell is to duplicate any other spell of 8th level or lower".
2. When a spell is upcast - it becomes that level. So a Fireball cast at 8th level is an 8th level spell.

Basically if you wish for a 3rd level fireball, than it counts as a 3rd level slot.  If you wish for an 8th level fireball, it counts as an 8th level slot.