What's wrong?
It sounds like you're using flat/mirror reflections, but you also mentioned
that you've adjusted the refmap size to 1024. That refmap size field is
ONLY applicable for cubic refmaps… not mirrors… since mirrors are
always made at screen resolution.
What antialiasing settings to you have set in that dialog box? If refmap
AA is set to none, you'd have plenty of aliasing in the mirror.
– G