For high rise building, the dimension of water tank along with drawings are provided from architect office.
But for small residential building, we need to calculate by itself.
VOLUME OF WATER REQUIREMENT:
As per IS code, a person needs 135 litres water per day for daily use.
Drinking – 5 Litre
Cooking – 5 litres
Bathing and Toilet – 85 litres
Washing clothes and Utensils – 30 litres
House Cleaning – 10 litres

CALCULATION OF TANK SIZE AND CAPACITY:

We will calculate for a typical family having 4 members.
Total water requirement = 135 x 4 = 540 litres
To calculate the size of water tank we must know any  one of the following:
Length or width or depth of the tank.
From the formula of volume of water
m3 = 1000 litres
1 litre = 0.001 m3
We are requiring 540 litre water
 540 litres = 0.54 m3
Let us assume the depth of water tank is 0.6 m.
 Area of tank = 0.54/0.6 = 0.9 m2
L x B = 0.9 m2
Now if you know the length or breadth you can put the value on the above formula.
otherwise, take the length as 2 times of B.
 L = 2B
2B x B = 0.9 m2
B2 = 0.45
B = 0.67
 L = 2 x 0.67 = 1.34 m

So For 540 litre water tank size
Length (L) = 1.34 m
Breatdh (B) = 0.67 m
Depth (D) = 0.6 m