The Pixel Resolution setting sets the axis step over and step ahead in the file, so when resizing your image in the Editor, use the Auto option with the same Pixel Resolution setting as on the main screen. Instructions are on page 8 in the Tutorial.
As far as the line count, 15" X 23" is a large image and when there are short incremental raster moves, the line count will be very high. If you have pure white areas around the border of your image, select the Skip Repeating White Colors so PEP6 will ignore generating the gcode in those areas.
Our PicSender gcode streamer was developed for running the fast changing, very large line count raster gcode files. I have tested PicSender running a 20,000,000 line raster gcode file on my Win 10 Dell Pentium 3 Ghz 4 GB Ram PC. It ran non stop for 14.5 Hrs.
Give us more detail of the image (B&W Logo or Color/Grayscale) your wanting to engrave and if it's with a spindle or a laser.