I have Shar Jorgenson's (Quilting from the Heartland) apple core, it's approximately 4.75 inches long, and 2.75 in the cores. She has layouts for large quilts but not really a chart to do for sizing, but for approximating two together in a T shape would be roughly 7" (4.5+2.5, taking off seam allowances). The final row size with alter a little after that on whether you have an inny- or an outie- row.
I'd measure your template, figure out your two-piece section and figure your length and width that way and that it would be close enough. No borders typically for an applecore...