Hi, it basically comes down to what size pieces you are cutting? If you are wanting to cut pen blanks, I'd make it so that the mitre slot was on the edge of your piece of ply and the other end long enough to be able to support the longest blank that you would want to cut. I use a bandsaw for all my cutting and have made it, so that I can cut blanks full length plus a bit for trimming, then reset the stop to cut for the next size or to length for 2 piece pens. Most are made quite small as I don't have much room to store them. I have them to suit a lot of different angles, etc. I think from memory I have about 20 different cutting sleds, from circle cutting to square cuts for blanks, cutting dowel even one for cutting round logs to square, all are adjustable for different lengths.
Kryn