Hi vijay,

The size of beam, column and MS Bars depend on load coming on the beam and column. Sizes are calculated by structural engineer and details are given in structural drawings. However for general building we keep the depth of beams under the slab 1 inch per RFT length of beam i.e. if we want to lay 15 feet long beam, the depth of beam under slab may be 15 inches and total depth including slab may be 20 inches. The width of beam should be according to the width of column below beam. Generally width of the beam is 9 inches or 12 inches. In case where column is not provided, RCC bed plate must be laid under beam and width should be 12 inches.
The size of column where the space between columns 9 feet to 12 feet the width of the column may be taken as from 9 inches to 12 inches and depth from 12 inches to 15 inches. Placing of MS steel Bars is following.
4. For length up to 10 feet = 4 Numbers 12 mm and 2 number 10 mm steel bars.
5. For length up to 15 feet = 4 number 16 mm and 2 number 12 mm steel bars.
6. For length up 20 feet = 4 number 20 mm 2 number 16 mm
Rings of beam should not be less than 8 mm diameter bars and 4 inches center to center lengthwise. The steel bars in column may be 6 numbers 16 mm dia bar for general loading.  If the load is more, increase the dia of the bars.
Caution
1. These sizes are according to cement concrete 1:1.5:3 ratios mix and for vibrated concrete.
2. The sizes given are for general building. If you want sizes for multi storey building, these should be taken from structural engineer.
 

Thanks