Size: The Size factor is easy to quantify. It says that small firms, over time, give higher returns than large firms. The size of the firms is typically measured by market capitalization. All other things being equal, what this implies is that small cap and mid cap funds and stocks should, over time, outperform large cap funds or stocks.
Some caveats here are useful. While the original research around factors showed a strong size effect, over time, many academicians have criticized the methodology of these original studies and disputed whether such a size factor actually exists. In India, moreover, we also have the challenge that it is sometimes difficult to trust the corporate governance in small firms.
So my suggestion is that instead of explicitly choosing small over big, it is better to select stocks from a broader benchmark (like the NSE500) than a benchmark with only large cap stocks (like the NSE 50). Once the broader benchmark is selected than you can tilt it towards the other factors as mentioned below. This ensures that you are choosing stocks with a tilt towards smaller stocks without explicitly pitting off small against big.