If we are using a fixed number of digits to make 2 numbers that when multiplied with one another gives us the max product, then the 2 numbers should have same number of digits or something closer. Ie. In case of 1, 2, 3, 4, & 5 we must have one 2 digit number and one 3 digit number since we cant have 2, 2 or 3, 3 digit numbers here.

It can be found that 52 × 431 = 22412 is the greatest product.