The absolute minimum for an adult is a 150g, IMO.
I totally agree...the bigger the better. Tesselatas can max out near 6 feet. Which is not to say it WILL get that big, but it CAN get that big. Plan on a minimum of 4 feet full grown, and THICK.
They generally are kept in species only tanks as they have a knack for eating anything, including other eels, that will fit in their mouths
I keep mine with a blackedge, and have had no problems thus far. There is a possibility of tesselatas eating other morays, but it's not the rule. Now as I write this, I'm crossing my fingers!
Don't make the mistake I did and keep a fish in there with it. I had a HUGE wrasse in there with my (medium sized) tesselata, he ate it in one gulp.
Also, if you get one, plan on keeping it. They're not easy to unload if you decide you don't like it. My LFS has one that was traded in a year ago, and it's smaller than mine. No one is ever interested once they learn about them.
And of course, make sure you can totally seal up your tank if you get one. They will try to liberate themselves given even the smallest opening. They also have a habit of rearranging their rocks regularly.
Other than that, they are AWESOME, spectacular looking eels!