I have a .224 blank that is 20 inches in length. It has a 1-9 twist. I would like to know the lightest bullet that this fast twist barrel will shoot. Any thoughts?

Are you thinking of building a 22Hornet or a 22-250AI?
Twist rate is only 1/2 of the equasion, MV is the other half.

I was thinking about a 22 BR.

Once again, it isn't the weight, but the length.

A 1:9 twist in a .224 barrel, at velocities under 2800 FPS, will be optimal for bullets 0.793" to 0.882" in length.

At over 2800 FPS, your 1:9 twist is basically useless.