Where does 2525 and 252525 come from in RSA cryptosystem example?

Question

This is an example from Discrete Mathematics and its Applications

I understand how to encrypt, the first step is to turn the letters into their numerical equivalents(same thing we had to do for shift ciphers). The author's justification for grouping these numbers into blocks of four digits is that 2525 < 2537 < 252525.
I understand what n is. n in RSA is the product of two primes, 43 and 59 in this case. But why does n have to fall in the range of interval notation (2525, 252525)? Where do these two integers 2525 and 252525 come from?

Answer 1

I suppose it comes from the encoding procedure of having A - 00, B - 01, ... , Z - 25.

Answer 2

Each letter generates two digits in the range $00-25$ Having selected the modulus $2537$ we cannot encrypt any number larger than that. We note that the four digit codes coming from two letters cannot be larger than $2525$ and $2525 \lt 2537$, so we can encrypt any two letter pair at once. Since a three letter set can generate a code of $252525$ and that is too large, we choose to encrypt letter pairs as the most efficient. If we had chosen a product of primes in the range $252525-25252525$, it would make sense to encode letter triplets.