I don't like "realistic" dildos. Amputated/free floating wang. Bleagh.
The largest toy I used had an 8" insertable length and a circumference of 8" at the thickest part. Because the toy was a somewhat squishy silicone the "extra" length that wouldn't fit straight on ended up partially bent inside of me. I can accommodate and enjoy more length than (apparently) a lot of ladies, but 8" won't fit. The toy was flexible enough I could have tied it in a knot if it hadn't been such an odd shape, so that's why the tip/head of it was able to curve/bend to the side.
A lot of the really big toys I've seen at generic/main stream toy stores aren't made of a material I would want to put in my body. The best place I know of for HUGE toys is Bad Dragon. They cater to furries and most of their toys are not human shaped. The quality of them is amazing, though.
I would get your girlfriend's input before buying anything. I've talked with my partner enough that he knows my likes/dislikes, so he could buy me something if he wanted. I also used to write product reviews for dildos, lube, and various other adult products. I have very, very, very specific demands. Absurdly large dong or not, if you haven't already gone toy shopping with your lady, the most important suggestion I would make is to get her input and/or take her shopping with you.
The overwhelming majority of women I know rank girth over length for their pleasure. I like a certain amount of length with the dildos I buy, but past a certain point length literally will not fit inside my body. So unless you're wanting to hold the base/extra length of the toy that won't fit inside her body with both of your hands.................
Anyway, fap fodder/fantasy whatever your thing is, that's my input. Truth as I know it and genuine.