Just made the jump from a 32" CRT to a 60" HDTV and agree with Sitlet that HD channels look great but SD channels look horrible especially because my screen is so large. If everything they watch is available in HD then no problem and you can get as big a screen as you want. If some of their favorite channels are only available in SD (like me) then maybe you might want to get a smaller screen under 40". By the way if the old CRT is 27", you need to get at least a 34" HDTV for the SD channels to "pillar box" to the same size as on the CRT.
With my cable service SD signals are either shown as "pillar box" or "postage stamp" when I set my TV screen setting to "Screen Fit" or "16:9". WIth these settings you are only filling up a portion of the screen but that's the only way to get at least a viewable feed. If you use something like "Wide Fit" or "Screen Fill" the whole screen will be filled but the picture quality is horrible, especially on a big screen, and you also lose content around the edges.