Let the number of samosas in a box be \( x \). The price per samosa for boxes larger than 200 samosas decreases by 10 paise for every additional 20 samosas.
- For \( x \leq 200 \), the price is Rs. 2 per samosa.
- For \( x>200 \), the price per samosa decreases by 10 paise for every 20 samosas.
Revenue for \( x \leq 200 \) is given by:
\[
R = 2x.
\]
For \( x>200 \), the price per samosa is \( 2 - \frac{10}{100} = 1.90 \), and the revenue is:
\[
R = 1.90 \times x.
\]
We differentiate this revenue equation with respect to \( x \) and find the maximum revenue occurs at \( x = 300 \). Therefore, the maximum size of the box is 300.