You need to take measurements once the fan is down. Most holes will be 4" in diameter, but the depth may vary. Some installations use a shallow outlet box (mine was 1/2", the depth of the sheetrock), but I've had some that were the deep type. Keep in mind that some ceiling fans aren't mounted directly to a joist. I used a stud finder to make sure there was a joist before I removed my fan. I used a 3.75" x 2" x 1/2" piece of hobby wood from Home Depot. It was a super-hard Poplar wood, I think. Home Depot also had Oak. They probably have Pine as well in the regular section, but I didn't want to buy 7' or 8' of wood. The Poplar piece was 3' or 4' long if I remember correctly. We had to cut the piece to size; 3.75" was the perfect fit even though the diameter was 4" because of the arc of the hole. I then covered the hole with a decorative piece of wood that I painted a shimmery white color. Had to get the glitter in somehow! Unfortunately, you can't appreciate the shimmer unless you get close :-/
Once I get my SV pole (Wednesday, I hope), I will post pics and a review! I obsessed about the hole in the ceiling for a few days since I wanted it to look nice. Also, my fiancé probably wouldn't like it to look unfinished! The covers in the home improvement stores were not suitable as they did not have a hole for the pole. I guess I could have drilled a hole, but it was still an ugly cover lol. I think I found a reasonable solution, and it wasn't expensive :)