The luggage allowance for flights to the Galapagos typically includes one checked bag up to 50lbs (23kg) that must be no more than 158 linear cm (62.2 in), adding height (A) + length (B) + width (C). Plus one carry-on up to 22lbs (10kg), and a small personal item.
However, if you are on an island hopping tour or self-guided visit, the smaller your suitcase the better. Moving large bags from island to island on taxi boats and ferries is not so fun, and the inter-island planes have lower allowances.