This is an interesting topic, as I have have recently decided to go back a little longer with my prone foil board. If you can set up your foil board correctly (mount in a balance position), length is not as critical as you may think it is given all the media out there riding tiny boards.
I am riding the Naish Ascend 5'0" x 22.25 @40ltrs. Whilst this board is still quite small, it is much larger than the 4'8" I was previously riding (and with no issues either) or the Hover 127 (4') or Hover 112 (3'8"). I am 90kg and been foiling now for about 2 years. I like and understand the idea of going super short, but the weigh offs need to be taken into account. Since changing back up to 5'0", my wave count and session length have increased again (like they used to be in the 5'6" to 6'0" days

). I think there is also a lot to be said about actually using your positive swing weight.
Again a lot of this trim-ability comes down to the use of the track and plate mounting, it is mandatory IMHO. 50-100mm can totally change a prone boards characteristics. Increasing swing weight by moving the backwards makes the board far easier to handle more powerful waves and turn harder, tighter and is especially good for hitting white water (round houses & floaters), whilst moving reducing your swing weight by moving the mount further forward makes the board far more responsive to pumping and very lift positive (i.e. you will feel like you're keeping it down as opposed to bringing it up).
I usually say start at the back of your tracks and play as often as you can. I use a marker pen and put measures on my board and when I find a killer setup of a particular condition, I make note. This way I can enjoy perfection the next time I ride.
Anyway, board length. Go short by all means, just be aware of the weigh offs. I pump out, link up and can do everything as I could on a shorter board. Look at where you plan to ride, if it's a lot of paddling, definitely go longer (like the Naish Commet), if it's generally a short paddle with lots of pumping go a little shorter (like the Naish Ascend). Aim around your preferred fish or small wave board volume.
Ride safe,
JB