Through my experience looking into anal plugs there seems to be a pretty wildly varying concept of "medium" so I wanted to recommend a good ramping set of plugs for beginners.

Everyone's butt is different, however a starting gauge is the general bowel movement size of the person interested in anal play. If they have super small ones that look like a house cat, they are going to need more work than someone who leaves impressive things in the toilet. Toilet humor aside, no matter how one goes to the bathroom, if interested in anal sex I highly recommend plugs to ramp up to being able to enjoy anal sex.

To get things started, putting on a nitrile/latex glove and using a finger to lube and probe an ass is a great introduction. This lets the bottom get used to the general feeling of anal play and one can even scale up to using 2 fingers once they are comfortable. For a size just a little bigger than a finger I like the Cal Exotic Booty Call Booty Rocker plug which is what I'd call a true small. The tip is great for starting the insertion then expands enough to stay in despite not being very big (Approx 1" max width). As seen on all these plugs, the base conforms to the body rather than being some weird end shape. This is my favorite end style since it allows one to sit down with the plugs, also functions as something to grab to pull out, and still prevents the plug from disappearing in one's ass.

For those with tight asses, any sizes from here up are typically difficult so that is why I like the Cal Exotic Booty Call Booty Beads. Using 3 beads it is kind of like having 3 plugs that ramp up in one. The starting ball is about .75" wide, middle is 1", then the last one is 1.25" ish. This way you get a small ball for insertion, middle ball to make it to earlier progress, and the last ball to gain a little in size. Ramping up further to get to something more cock-like, I think the Tantus Neo is a good fit. Very simple in shape, it maxes out at 1.5" width which is getting closer to an average cock size. From there I've always had good luck transitioning to anal sex, but don't be in a rush to do all of this in one session. Over doing it and causing an anal tear is a good way to make a beginner have their mind set that they don't like anal sex and never want to do it again. Thus up to us good dominants out there to give good anal experiences to create more anal sluts :)

Just in case it was not obvious, lube is needed for any sort of anal fun as the ass does not self lubricate like our sexual organs do. One for the most part can't over-do it on the lube, the only down side to using a lot is a higher chance of making a mess. For ease of cleaning and being the most friendly with all kinds of materials I recommend water based lubes like Sliquid Sassy Lube. Also, all these plugs are silicone based as opposed to being crap jelly materials that can be harmful. This fact along with being fairly affordable makes them more reasonable to use without a condom and to have each only used on a specific person.

Small Starter:  Cal Exotic Booty Call Booty Rocker

Medium:  Cal Exotic Booty Call Booty Beads

Bigger: Tantus Neo Anal Butt Plug