The Wide-field camera 3 on Hubble has revolutionised the study of the high-redshift Universe.  With deep surveys providing high-resolution data in the near-Infrared (CANDELS and UDF12/HUDF09/XDF), we have been able to re-examine the sizes and morphologies of galaxies at this epoch.  These data are used to compare to theories of disc formation and evolution, so I will give an introduction to the simple analytical theory used. Previously, the evolution in measured sizes (from z=7 to z=4) were attributed to the build-up in the population of large galaxies at lower redshifts. Our results suggest that limitations in surface-brightness dimming and angular diameter distance changes are still affecting our ability to accurately measure the underlying size distribution at these high redshifts. In fact, taking account of these effects we find very little evidence for strong size evolution at these epochs.  I will talk about the relevance of the theory to analysing galaxy samples at these epochs.  Can all of these objects be relaxed discs?  Could measuring sizes from the rest-frame UV affect the measured size evolution?  Are there any processes that could both affect measured sizes and UV luminosities and can we use the morphologies of these objects to infer if there is any evidence that they are shaping the measured size distribution?