Exactly. Single layer discs hold 4.7GB, which is enough for, well, about two hours (in standard MPEG-2 format - if you're only playing the disc on computers, you can use h.264 to put in about the same amount of material in HD, which was one of the reasons to not switch to a blue ray disc format). Doulbe layer discs hold 8.5GB, about twice as much.
They usually say on the case if they are dual layer or not, most are.
One thing you could do is make an ISO of a dual layer disk in iMovie and then shrink it with one available tools to fit a dual layer disk on a single layer disk and burn the resulting ISO with DiskUtility.