I recommend starting small and having a ramp plan with beginners. I’ve found this to be a good ramp progression:
Cal Exotic BOOTY CALL BOOTY ROCKER BLACK
California Exotic Novelties Booty Call Booty Beads, Black
Bigger: Tantus Neo Anal Butt Plug, Black
For tails the crystal delights plug tail size is a good smaller size that is still big enough to stay in:
Comfort is of course going to be relative when it comes to anal :) but all of these designs are smooth without any extra irritating features.