Cell size is partly determined through coordination betweencell growth and division. How this coordination is achievedmechanistically remains mostly unknown. However, quantitativeexperiments together with computational modelling havereinvigorated the field and are elucidating underlying molecularprocesses. Size homeostasis may be achieved throughdifferent modes of regulation, including “sizers”“adders” and“timers.” For sizer regulation, the cell division cycle does notproceed until a minimal size has been reached, requiring thatthe cell monitors its own size. Here, we highlight progress indefining sizer mechanisms in fission and budding yeastsshowing how accumulation or dilution of key molecules can beused to monitor cell size during growth. We also discuss apotential role for sizers in bacterial size control.