With a male, you can get away with a 4ft tank (120cm) ....but only if the water is very deep and tank is not narrow. A 5ft is ideal and a 6ft is excellent. Even a baby needs at LEAST a 3 ft (90cm) tank and that should be changed to a 4ft (120cm) by the time the turtle is a year old. To save money its best to get a larger tank to start with! Photo is of a 6 ft tank. The bigger thetank the better. A large external filter is needed as turtles are very messy. Partial water changes are needed every week to stop turtle living in their own filth. Shingle is a huge killer of turtles... so plain tank bottom is best. Driftwood is good for them to scratch, chew, rub against to helpremove scutes etc. Rocks can lead to ulcers, crush injuries and possible drowning. Need 2 lights. A UVB reptile light ... if it doesnt say reptile, its no good. You also need an ordinary incandescent bulb or a reptile heatlamp for a 'heat' source to encourage turtle out of the water. UVB bulb needs replacing 6 to 12 mthly. 
(Although there are several turtles in this tank, they are young and physical conditions due to vitamin deficiency has led to deformed mouths etc so they are not bullying or fighting. BUT they are carefully observed and will be seperated if required. It is best to have oneturtle per tank to prevent fighting and injuries.) | 
