Does size matter in case of attack on LUKS partition? Does the size of the LUKS partition affect the difficulty of a brute force attack, or would it not make difference? For example, would a 100MB partition be easier to attack than a 100GB partition?
No, the size does not matter. No matter which algorithm you use, the attack with the best chance of success is to guess the passphrase, unless the passphrase is a long, random, unmemorizable one (i.e. a keyfile in practice).

Attacks against the passphrase are not helped by a larger amount of ciphertext. They work by generating the key corresponding to the passphrase, and then attempting to decrypt with the resulting key. A few bytes of ciphertext are enough to validate the guess.

If the algorithm is the limiting factor, then finding the 128-bit key using the fastest known techniques requires very close to $2^127$ attempts on average ($127$ and not $128$ because on average the key is found after exploring half the search space). Knowing a lot of plaintext-ciphertext pairs does not help to find the key or to decrypt other ciphertexts: cryptographic algorithms are designed to resist such attacks.