I wouldn't start with either of those, I'd start with a 1.5-1.75" plug to warm up and then go up from there. I do a little thrusting with them to make sure I'm relaxed. My order for A-Bomb is Severin Medium, Sasha Grey Large, A-Bomb.
Use of an enema depends on the situation. For most plug use you can go to the bathroom and then do a quick check with a finger or dildo because plugs don't go beyond the lower rectum. If you're planning on moving to deeper play or feel you need to be cleaner (either because of a dirty check or because you just know your body or partner) you can use a water enema, but avoid chemicals and keep it as small as you can while still having it work.
One other pointer - warm up the plugs before use, because it makes insertion much easier.