SRB-A engine effects don't scale properly with booster size, shrinking to 0.625m diameter causes the effect to become very large and move up above the nozzle. Increasing booster diameter above 1.25m causes the effect to shrink and move down below the nozzle, the larger the diameter the farther below the nozzle the effects move, it is hard to tell if the particle size gets smaller and smaller with increasing diameter though.

Repro:
-make craft with pod and SRB-A
-Change SRB-A diameter and launch, observe improper effect scaling

Possibly related to issue #823 ?

Log: KSP.log