My blunt penetration capacity is nearly identical to alan20cm's report. I don't get the pain he does from a wide toy but do trigger a lot of upper colon peristalsis and gas generation. That leads to spasm and an absolute brick wall. After many years of play, I've accepted that I'm not built for long and 'girthy' penetration. It's a matter of what I call the "discomport to pleasure ratio." :D
My fr38 tube experience has been well across the transverse up to the hepatic bend. Soooooooo ...
To explore all the desights of my rectum though I took a short, closed-end rectal tube and stiffened all but the last two inches with a piece of #10 copper wire. The unstiffened tip beyond where the wire reaches has been a nice explorer. I can form the tube to various shapes. It's versatile and for sensory explorations has outdone my commercial dildo. Like any colon tube, it's easy to go through the sigmoid sphincter. So, there are few limits on 'distance'
I like a shea butter or a thin oily lube for these ventures. For me, the better (thicker) water based lubes seem to mask the more subtle feelings.
If you fabricate one of these, make sure you've got something like a right angle bend in the wire and the lower end of the tube, so that you know, at all times, where the end of the wire is in relation to the opening in the tube.
Another feature. When I wish to inject some lube behind a fold or in a distant spot, I just pull out the wire and use a lube shooter to inject the lube. Safety tip: I never re-insert the "insertion" wire into an already inserted tube. I pull out the tube, reinstall the stiffening wire then slide the whole business back in.