I learned the use of 크기만한 to mean "about the size of" and 만큼 to mean "as much as". Are the following both correct?
모기 크기만 한 벌레, O
bug about the size of a mosquito
모기만큼 큰 벌레, O
bug as big as a mosquito
1) Is there any difference? I see they both use 만 (only) and a noun form of 크다 (to be big): 크기 and 큼.
"모기만한 벌레", in '만 하다', '만' is used as a auxiliary particle for showing "having/regarding as much degree or amount as that(noun) of quality, size, etc.", after a noun usually. And as you know, '만' means "only" as well.
When you study the particle '만', you could refer to "만하다(auxiliary adjective") or the bound noun '만'. So, You would be very confused between them.
In addition, 만큼 is used as either a bound noun or a particle. In this case, 만큼 is used as a particle, showing that "as much degree or amount as "
2) Can 만한 be used with other nouns, for example the following:
모기 질량만한 벌레
bug about the mass (weight) of a mosquito
Sure, It could be. But, It is better "모기만큼 무게가 적은/질량이 작은 벌레" or "모기만큼 가벼운(light) 벌레", and you should distinguish between 무게(weight) and 질량(mass) in physics.