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

formofphotoelectricandphotoionizationheating(Abbott1982;Joung&MacLow2006),andamodelforshieldingofUVradiationbyneutralhydrogen(Cenetal.2005).Theyalsoincludemetallicity-dependentradiativecooling(Cenetal.1995)includingmolecularhydrogen(Abeletal.1997)andmetalcoolingextendeddownto10K(Dalgarno&McCray1972).StarformationandsupernovaefeedbackaremodeledfollowingCenetal.(2005)witheSN=3×10 6.StarparticlesarecreatedincellsthatsatisfyasetofcriteriaforstarformationproposedbyCen&Ostriker(1992).Eachstarparticleistaggedwithitsinitialmass,creationtime,andmetallicity;starparticlestypicallyhavemassesof～105M⊙.Feedbackenergyandejectedmetalsaredistributedinto27localgascellscenteredatthestarparticleinquestion,weightedbythespeci cvolumeofeachcell.Themetalenrichmentinsidegalaxiesandintheintergalaticmediumisfollowedself-consistentlyinaspatiallyresolvedfashion(Cenetal.2005).

WeidentifyvirializedobjectsinourhighresolutionsimulationsusingtheHOPalgorithm(Eisenstein&Hut1998),whichistestedtoberobust.We nd49haloswithvirialmasses>5×1010M⊙aspotentialLymanbreakgalaxy(LBG)hosts(Somervilleetal.2001)andchoosethemostmassivegalaxieswithinthevirialradiiofthesehalosasLBGcandidates.

ThelightdistributioniscomputedfromthestarparticlesusingtheGISSELstellarsynthesiscode(Bruzual&Charlot2003).WefocusontheluminosityintheSDSSg′-band

[notethattheG lterinSteidel&Hamilton(1993)isnearlyidenticaltotheg′-band],correspondingtorestframeλeff≈1200 A.WeadoptanapproximatemodelfordustextinctionfollowingBinney&Merri eld(1998)butassumethatdustattenuationis

proportionaltothemetalcolumndensityratherthanthetotalhydrogendensityandcorrectfordepletionofrefractoryelements(Zn)ontodustgrainsparametrizedbyfFe,fractionofironindust(Vladilo&Peroux2005):AV=ΣZfFe/(4×1019mpFcm 2)mag,where