Using state-of-the-art adaptive mesh refinement cosmological hydrodynamic simulations with a spatial resolution of 0.15 h-1kpc on a cosmological volume (20 h-1Mpc), we investigate the sizes of galaxies at z=3 in the standard cold dark matter model where re

Sommer-Larsen&Dolgov2001;Bode,Ostriker&Turok2001)

Sincetheageoftheuniverseatz=3isonlyabout1/6ofthepresentage,asuccessfulresolutiontothegalaxysizeproblematz～3mayprovideimportant“cleaner”cluestothenatureofthedarkmatterand/orimportantastrophysicalprocessesathighredshifts.Moreover,combiningobservationsatbothz=3andz=0mayprovidestillmorepowerfulconstraints.

WethankDrs.Yen-TingLin,Mordecai-MarkMacLow,andAliceShapleyforhelpfuldiscussions.Wegratefullyacknowledge nancialsupportbygrantsAST-0507521andNNG05GK10G.