Measured Room

Ability Score Improvement:   +1 to INT, WIS or CHA.

They may call you unsubtle, maybe even crazy, but they can't say you're not trying to be careful... sometimes.

You always have the Fireball spell prepared (cast using the same Ability Score you increased with this Feat) and can cast it (Level 3) without a spell slot once per Long Rest; You can also cast it with any available spell-slots of 3rd level or higher. Whenever you cast Fireball you may choose the sphere-radius size (in increments of 5ft) from 5ft up to 20ft.

At Higher Levels: When you cast Fireball with a spell-slot above 3rd, the maximum chooseable radius-size of Fireball's sphere increases by 5ft, (Level 4: Up to 25ft, Level 5: Up to 30ft and so on) 

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NOTE: This is the post contest version of the Feat. For the version that (I hope) went before the judges, see Version 1.1 (I think; the numbers seem to have gotten away from me...)