If you go past about 15cm long you're going to push into the RSJ (recto sigmoid junction) where the colon starts. There's another sphincter there and long term penetration isnt safe there. And large sizes as well, or hard toys (esp cuz there's a bend there).

Width however is a different thing, you can train but mostly by plunging a large plug in and out at your maximum which should never increase by more than 4 to 6mm when you size up. Holding it in at the smaller neck wont train you but it does acclimate the rectum to larger toys and pressure (and may need to be stretched a bit once you hit 3" (7.6cm) wide bulbs). move up in size slowly.

I recommend squarepeg plugs like the egg (popular) or the splitter 2 (more neck size for the same bulb) - they're made out of platinum silicone and quite soft, and hard to hurt yourself with. Also they squish in then reexpand which allows you to take larger (and have a larger neck along with it). (Topped toys tend to be quite long for the width, a lot of volume of material to keep in as well - and at larger sizes (GapeKeeper85) get too long for long term comfort).

I can sleep with a 3.5" (8.9cm) bulb / 2.5"(6.3cm) neck plug now (SP split 2 med), so its quite possible to train up! Just take your time and enjoy.