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The exploration of the relation between galaxy sizes and other physicalparameters has provided important clues for understanding galaxy formation. Weuse the CANDELS Deep+Wide surveys in the GOODS-South, UDS and EGS fields,complemented by data from the HUDF09 program, to address the relation betweensize and luminosity at z\sim7. We select 153 z-band drop-out galaxies in sixdifferent fields characterized by a wide combination of depth and arealcoverage, ideally suited to sample without biases the observed size-magnitudeplane. Detailed simulations allow us to derive the completeness as a functionof size and magnitude and to quantify measurements errors/biases. We find thatthe half light radius distribution function of z\sim7 galaxies fainter thanJ=26.6 is peaked at <0.1 arcsec (or equivalently 0.5 kpc proper), while atbrighter magnitudes high-z galaxies are typically larger than ~0.15 arcsec. Wealso find a well defined size-luminosity relation, Rh\simL^1/2. We compute theLuminosity Function in the HUDF and P12HUDF fields, finding large spatialvariation on the number density of faint galaxies. Adopting the sizedistribution and the size-luminosity relation found for faint galaxies at z=7,we derive a mean slope of -1.7\pm0.1 for the luminosity function of LBGs atthis redshift. Using this LF, we find that the amount of ionizing photonscannot keep the Universe re-ionized if the IGM is clumpy (C_HII>3) and theLyman continuum escape fraction of high-z LBGs is relatively low (f_esc<0.3).With future CANDELS data, we can put severe limits to the role of galaxies inthe reionization of the Universe.

The size-luminosity relation at z=7 in CANDELS and its implication on reionization