Cell size checkpoints ensure that passage through G1 and mitosis occurs only when sufficient growth has occurred. The mechanisms by which these checkpoints work are largely unknown. PP2A associated with the Rts1 regulatory subunit (PP2A(Rts1)) is required for cell size control in budding yeast, but the relevant targets are unknown. In this paper, we used quantitative proteome-wide mass spectrometry to identify proteins controlled by PP2A(Rts1). This revealed that PP2A(Rts1) controls the two key checkpoint pathways thought to regulate the cell cycle in response to cell growth. To investigate the role of PP2A(Rts1) in these pathways, we focused on the Ace2 transcription factor, which is thought to delay cell cycle entry by repressing transcription of the G1 cyclin CLN3. Diverse experiments suggest that PP2A(Rts1) promotes cell cycle entry by inhibiting the repressor functions of Ace2. We hypothesize that control of Ace2 by PP2A(Rts1) plays a role in mechanisms that link G1 cyclin accumulation to cell growth.