Single cots are generally 6 1/4 feet long. The width can vary from 2 feet to 4 feet, though it is generally common to see a 3 feet wide cot. That said, the length could also be 6 1/2 feet, although this is less common at least in India. Mattresses are available in this size.

If you are 6ft tall, then you need to 61/2 ft mattress, cot. I had to custom ordrer my cot, although the mattress was available.

Now converting those to cms,
6 1/4 ft ~ 190 cm
6 1/2 ft ~ 197 cm
3 ft ~ 90 cm

I have seen double beds in two different sizes.
1) 6 1/4 ft * 5 ft – this is generally enough and in most modern dat apartments, you will find space enough only for this.
2) 6 1/4 ft * 6ft – this is equivalent of two single cots put side by side. The advantage with this is that you could perhaps use two single mattresses if you already had them, though it is not really such a great idea.

Single Bed 30″ x 72″ (2.5×6)
Medium Bed 36″ x 72″ (3×6)
Double Bed 48″ x 72″ (4×6)
Triple Bed 60″ x 72″ (5×6)
Maharani Bed 60″ x 78″ (5×6.5)
Maharaja Bed 72″ x 72″ (6×6.5)