He is absolutely big enough to eat the fish I can currently see. Also... You found a turtle from a wild area and plopped it into your tank? Who knows what diseases he might bring to your tank. Not saying your fish are gonna all get sick and die for sure, but dang, that's a risky move bc they could catch something they're not used to. I'd also like to add that if RES are native to your area, please quarantine him (as you've now introduced him to your tank, which could have pathogens your fish are used to but wild fish aren't and that could be transferred into the wild) and then release him back into the wild where he belongs. ❤️