I understand what you mean, I can only take so much up there too. With my anal dildo I can't physically take the whole length. Its only about 8" or so. Same with my vagina, I can only take 7.5" before I can feel it reaching my cervix! It does fascinate me how you see women in porn taking upto 20". Perhaps we're just not anatomically built for it 
Have a look at some anatomy diagrams. Also the entrance to your rectum is sort of closer to your back than your front so the toy needs to be angled in the right way to enter the pathway from the rectum to the colon. The toy that you linked to is unlikely to actually be long enough to get to the bend (sigmoid process) in all honesty, and most people could probably insert the full length but remember when playing around with depth, your colon and intestine does not have pain sensory nerves, so when you feel pressure inside that is an indication to take it easy before your rupture something. Also for depth play, think about how much lube you would normally use and triple it and it needs to be applied inside so get a lube launcher.
An enema can be a very good way to prepare for depth play, it both cleans you out and warms up your sigmoid sphincter as well.
I don't know how big big is but I play with a few large, girthy guys at the moment and, it sounds weird, but taking their actual member has always been easier than toys. I don't know why but just seems that way.
What I have found useful and works with my primary partner when I'm pegging too is using a butt plug thats too bloody big, or trying to but stopping when it's painful. The one we use is on sale! https://www.lovehoney.co.uk/product.cfm?p=30702
By using this we are as wide as we're frankly going to get so lube and being gentle finishes the job :)
P.S
From my experience the trouble is girth more than length so I would personally focus on that. Good luck!
Even with very flexible toys I find it can be tricky to get more than 8 inch in say, I found sometimes rather than push the toy straight up (say when standing ) if I used it more angled up at rear down at front it slips past the "choke" point then will easily slide in to about 16" I had a 22" double very flexible dildo but could only ever get it to 18/19" then my body said stop, it's tip was on my left side level with my belly button, never did get further.
I think it is very much a matter of relaxation, lots of lube and taking things slowly. I think (unlike for shorter penetration into the rectum only) that you do need an enema to prepare for taking something up into the sigmoid colon. I found that repeatedly taking the dildo up to the point where significant resistance is felt, rotating the dildo slightly (say 45 degrees each way) while keeping a modest inward pressure on would eventually cause it to pop past the obstruction.
The sensation of having a dildo that far inserted is strange - it seems to create a pleasant fluttering feeling that is absent with shorter penetration.