There may not be a giant rectangular hole where the flourescent light is placed as it does not take a large hole for the wiring of a flourescent light to go through. If by chance there is a larger hol than the new light fixture requires, you can use a ceiling medallion or if larger than that, you will need to install drywall or if in a home where beadboard would be appropriate, you could cover the ceiling with beadboard (country or casual look).
Thanks! The medallion is a good idea.
Typically the fixture will not leave a giant hole but your ceiling will need painted to match your existing ceiling. The wire that powers this light usually comes through the drywall without a mounting box and not in the center of your fixture. An electrician can install the proper box for your new fixture and move the wire to the right location.
This is what I did with the light opening left after I removed my flourescent lights in the kitchen. It's not a great picture, but you get the idea. You could add some darker accent paint to show it off.
As @Kimberly Barney stated a medallion would cover the small hole the wire probably came out of...or maybe the size of the canopy mounting on the pendant light would cover that hole. Your paint, where the rest of the fluorescent fixture covered, will be different than the rest of the room. You could do, like @Kerry did and it looks excellent, something really fancy and a bit difficult (although it looks GREAT I'm a little intimidated to attempt such a fine feat diy) or how about doing a kind of square/rectangular wainscoting on the ceiling using some type of molding to isolate that area and painting the inside a contrasting/decorative color/pattern for a unusual look? Jus sayn....