First of all the size of the columns depends on the total load on the columns.
- Minimum size of an RCC column should not be less than 9″x 9″ (225mm x 225mm) with 4 bars of 12mm Fe500 Steel.
- Use Minimum M20 Grade concrete in RCC column (ratio 1 part Cement: 1.5 parts Sand: 3 parts Aggregate with 0.5 parts water by volume)
- With thumb rule a column span of up to 5 meters is quite safe.
- Column longitudinal bars should not be less than 12mm and also its should be not more than 50mm by thumb rule.
- For using steel in column the cross-sectional area of longitudinal steel in a column shall not be less than 0.8 and also its should be not more than 6% of the gross sectional area of the column.
- I recommend use of 8mm stirrups at a distance of 150mm center to center throughout the length of column.