So…

(4/6)m >= (5/6)l
m >= (5/4)l

Which means Marty’s pizza is one and a quarter the size of Luis’ pizza. We can comfortably just compare the area, since we can assume a flat disk with equal height for a pizza.

Assuming Luis’ pizza is a Domino’s Classic size of 25cm that’s an area of:

(25cm / 2)² * π = (625cm² / 4) * π = 490.874cm²

So Marty’s pizza should be at least 490.874cm² * 1.25 = 613.5925cm² for 4/6 of his to be greater or equal of 5/6 of Luis’, so:

sqrt(613.5925cm² / π) * 2 = 13,975426964cm * 2 = 27,950853929cm

Since Marty’s pizza is equal or greater, let’s go with 28cm diameter… which happens to match exactly a Domino’s Medium size.

That’s a very realistic scenario and the teacher is an absolute idiot for not understanding.