This is such a sweet story about your 4-H advisor, still mentoring you!!
D9Ps can be made any size you want. Likewise lap quilt sizes vary considerably!
Consider the fabrics you want to use, to determine how big you want the squares to be.
Remember, that the centre one gets cut into four, so ends up considerably smaller.
(plus there is the seam allowance factor to remember!)
Many use 5" squares, but I found them too big for the fabrics I had and the look I wanted.
I downsized the individual squares and made a few sample blocks, trying different sizes.
Eventually I found what worked for the look I wanted.
Then, measure your D9P block, and calculate how many you need for your lap quilt.
Good Luck!