You can do this with TidyPlates,
Scroll down the TidyPlates : Damage Options and under "HEALTH" you can change the high health and low health treshold so unitplates change color between 100 - 80 %, 80 - 35 % and 35 - 0 %.
Then under the "SCALE" options change the unitplates to increase in size whenever a mob goes below whatever low health treshold you set.