Rank the following oxides in order of increasing aqueous acidity: Cs2O, Li2O, K2O. (From the least acidic to most - and please explain!)?

All of the three given oxides are basic because oxide ion can accept proton.

When going down a group, the atomic sizes are increasing because of the increase in electron shells.
All of Cs, Li and K are Group 1 (Group IA) metal. When going down Group 1, the order is Li, K and then Cs.
Atomic size : Cs > K > Li
Hence, ionic size : Cs⁺ > K⁺ > Li⁺

Consider the cation carrying one positive charge in each of the three metal oxide, i.e. Cs₂O, Li₂O and K₂O. A smaller cation would have a greater charge density. This would lead to a higher polarizing power to withdraw electron clouds from the oxide ion to a greater extent, and thus the oxide ion is less available to accept proton from water. In other words, the smaller the cation, the lower the basic strength of the oxide is.
Hence, basic strength : Cs₂O > K₂O > Li₂O

The higher the basic strength, the lower the aqueous acidity is.
Hence, aqueous acidity : Cs₂O < K₂O < Li₂O