The effect of Atomic Microsizer’s second ability will overwrite any previous effects that set the creature’s power and toughness to specific values. Effects that otherwise modify the target creature’s power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.
Equipped creature gets +1/+0.
Whenever equipped creature attacks, choose up to one target creature. That creature can't be blocked this turn and has base power and toughness 1/1 until end of turn.
