what a cutie.
i have 2 turtles 
Pooter (L) and Turtle (R)
i can try to answer your questions from the best of my experience
1)  I would say they are not much dirtier than fishies.  They take bigger craps thats for sure, but you can usually just scoop it right out.  Just like fish, you need to change their water out fairly frequently to make sure it doesnt get unhealthy.  I have a 40 gallon tank and I change the water about once a week.  My tank also gets a LOT of sunlight which accelerates the algae growth.  Im not really changing it so much bc it is dirty/poisonous, but bc it starts to get cloudy from the algae.  its not a health hazard, just not attractive looking.
2)  They are not really that interactive.  They will recognize you as their food source and will pretty actively follow you when they are hungry (all the time).  They are in general pretty dumb so they dont "play" with you.  
3) Different species of turtles eat different diets.  Some are more carnivorous that others.  I feed mine Reptomin sticks which they LOVE.  I give them lots of veggies too (carrots, anything green).  Somethings they like, others not so much.  but it is good to vary the diet.  Pooter doesnt really like the green stuff, but Turtle loves it.  They will also eat live fish and other meat...
4) I cant say for sure.  Depends on the species and sex.  I have heard different things about growing to the size of their environments.  Turtle may grow to be 10-14" (females are larger, and thats a large species).  Pooter will likely be a lot smaller (male, smaller species)
one thing to take into account is they need sunlight to keep their shells healthy.  natural sunlight is the best, but there are bulbs you can buy that give out the right wavlengths.  you can pm me if you want recommendations.
to the OP, be careful with the pebbles in there.  if your turtle can fit them in his mouth he probably will.  they can get lodged and cause serious problems.  most people i have talked to dont have any substrate in their tanks (myself included).  it gives them a little more room to swim and its also easier to scoop up the poops.