I prefer huge Sentinels because I want the mutants to feel overwhelmed by them. There should be a strong feeling of unity against the Sentinels, so mutants on either side(X-men, Brotherhood, civilian) have to work together to take them down for their own survival. It would make it even more impressive when single mutants take them down on their own, such as Magneto. Even mutants like Nightcrawler or Azazel can teleport top of one and bamf away its head. Mutants like Cyclops, Havok, or Banshee can rip through them with their beams/voice. Beast can use his brain and acrobatically maneuver from Sentinel to Sentinel as they destroy themselves trying to catch him. My point is that even single mutants could take down a Sentinel if their powers are strong and they use their brains, regardless of them being huge. However, uniting against them makes it much easier.