I built my turtle dock out of essentially Styrofoam and hydraulic cement and at its widest point is approximately 20" x13". It is not finished because I still need to cement the base on the bottom right side, as well as paint the whole thing with a waterproof epoxy resin. It will eventually go in a 65 gallon tank and will be attached to the right corner of the tank with aquarium silicone, which along with the base should provide adequate stability. I am in no way suggesting that you go my route because it can be very challenging. It takes a long time and it can also be hazardous if you're not very careful and do not do your research first.
The original piece was about 3 feet long. I sketched out a pattern and got to cutting.
I ended up with this shape:
This is what it looks like after applying the cement:
I'll post a picture of it in the tank with turtle once its finished.