I like the 26" myself though others have gone bigger, I think it's enough..the ITP steel 12x7, 2+5, 4/110 are perfect for the machine. Front and back.. Rocky Mountain has them for around $40/ea.

Then any tire is fair game since the 12" tires are very common..
Others may disagree but I think the Executioner 27x12x12 is over kill..not only is it over-size but it is an extremely HEAVY tire. 36lbs if my memory is correct..that extract weight combined with the extra circumference is gonna be a torque stealer!!
There is no end of combinations possible on the 250ES, have fun with it. This is just one guys opinion, there is no wrong answer!
Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)