Using wide-field , two-color HST imaging of the cluster CL1358+62 ( z = 0.33 ) , we derive structural parameters for a large , magnitude-limited sample of cluster members .
These structural parameters are combined with accurate velocity dispersions in another paper to investigate the fundamental plane in the cluster .
We fit integrated r ^ { 1 / 4 } -laws to the integrated surface brightness profiles , and fit two-dimensional r ^ { 1 / 4 } -law model galaxies to the images directly .
A comparison of the results from the two methods shows that the derived half-light radii , r _ { e } , agree very well , with an rms scatter of only 13 % .
The half-light radii range from approximately 1 to 20 kpc , with a median of about 3 kpc ( H _ { 0 } = 65 km s ^ { -1 } Mpc ^ { -1 } , q _ { 0 } = 0.1 ) .

We investigated the stability of r _ { e } by comparing the r ^ { 1 / 4 } -law fits to the half-light radii derived using other commonly used surface brightness profiles .
In particular , we fit Sersic r ^ { 1 / n } -laws ( finding the best-fit n ranging between n = 1 - 6 ) and superpositions of r ^ { 1 / 4 } -law bulges with exponential disks .
The half-light radii derived from the best-fit Sersic profiles varied with respect to r _ { e } from the r ^ { 1 / 4 } -law fits by only 1 % in the median , but with a standard deviation of more than 40 % in r _ { e } .
For the bulge-plus-disk fits , the derived half-light radii were offset from r _ { e } of the r ^ { 1 / 4 } -law fits by 10 % in the mean , also showing a large standard deviation of more than 40 % .
By comparing the fitted half-light radii from the Sersic laws with those derived from the bulge-plus-disk fitting , one also finds a large scatter of 30 % .
Based on these tests , we conclude that , in general , half-light radii are only measured with a typical accuracy of 30-40 % .

While there are large uncertainties in half-light radii , these do not impact the subsequent fundamental plane analysis because the combination r \langle I \rangle ^ { 0.76 } , which enters the fundamental plane , is extremely stable .
The rms scatter in r \langle I \rangle ^ { 0.76 } is less than 3 % , regardless of the form of the profile fit to the galaxies .

By fitting bulge-plus-disk profiles , we find that the median bulge fraction of the sample is 84 % and that the few E+A galaxies in this sample have disks which make up about 10-35 % of their total light .
These results are consistent with the residuals from fitting two-dimensional r ^ { 1 / 4 } -law models directly to the galaxies , in which disk-like structures are revealed in about half of the sample .
Two of the three E+A galaxies show spiral arm structure .