Yes, increasing the radius would allow for more movement, or you could use the alignment feature to align the images using larger apertures, and then run photometry using the size aperture you need to optimize photometric precision.
If all the images are plate solved, use the RA/Dec aperture placement option and you will not have to worry about how far the images move.
A third option is to use the variable size aperture. With that method, you can specify a larger max aperture size near the top (at the normal aperture radius setting control). Then set the variable aperture scaling factor such that you get the size of aperture that will optimize your photometry. In that mode, the aperture size used for photometry is the multiplication factor size that you set times the average FWHM of the stars in the image. Usually a value between 1.0 and 2.0 gives good results. I'd start around 1.5 and go from there to see what gives you the cleanest light curve.
Defocusing in general will improve photometric accuracy (to a certain extend), unless there are other stars that start blending into the aperture of your target star or one or more of the comp star apertures.
Karen
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