Unlike fish, turts continue to grow, regardless of tank size. Before I found this website, my RES grew from 7" to 11" in her tiny 55-gal tank. The good thing is that I was able to fill the tank all the way up to the top since my basking area was on TOP of the tank. But it was still too small for a full-grown adult RES, and just in the last 2 months, I finally received my new custom 90-gal tank for Sheba & she LOVES it. I definitely noticed a difference in her attitude right away. She no longer wants to get out of the tank all the time & is very content when she is in there, basking all day long & just hanging out.
Are you getting a heater for the tank too? You want to make sure the tank water is kept around 76-80 degrees for a RES. The submersible glass heaters aren't expensive (only around $20) and you can get a stick-on thermometer for less than $2. Although, I just noticed where you are from, and maybe you might not need a heater in Costa Rica! LOL! Hope you guys are having good weather there! I'm from Michigan & it has been snowing all day today & is very cold out (20 degrees F).
Have a super day!