Answer
The phenotype of Wee-1 mutant yeast cells would be tiny, which is the basis of the name "Wee."
Work Step by Step
Without Wee-1 protein, yeast cells would have cdc2 active all of the time, meaning that cells would rapidly pass through the G2/M checkpoint to mitosis. Cells would no enlarge during G2 as it would be shortened, and after each round of cell division, cells would be become smaller, more "wee."