don't use roll out icing, bloody nightmare. Use butter icing instead. Much easier to use and far more forgiving I'm just in the process in making DD (3 today)a 'princess fairy castle' cake for her party tomorrow and am hoping my butter icing will cover all the lumps and bumps
I got 4 small rectangular ones from Tesco once - it was clear they were cut from a larger rectangular one. We just pulled the lot out from the shelf and fitted them together like a jigsaw puzzle (on the floor, blocking the aisle!) until we'd got the best fit. Then I stuck them all together using jam. Then I cut out the shape I wanted, added more jam where it fell apart a bit and then iced the lot. It worked (just), but the kids didn't notice.