Don't know if parts editor will work (I expect not, but I don't often use parts editor). You (or someone) is more likely to need to modify the pcb svg file with Inkscape or one of the other editors. There appears to be a data sheet with dimensions for the pads here:
http://www.led-color.com/upload/201604/APA102-2020%20SMD%20LED.pdf
although it isn't particularly readable.
Peter