Some updated information was kindly supplied by Steve Ratzlaff.
The current batch of available Russian ferrite rods are 400 permeability. But the desired permeability for MW is ~ 125. Fortunately, according to Steve, The Australian Jaycar ferrite rod's UI = 120 specification corresponds to the more optimal ~ 125 permeability. Hence this requires more turns of Litz wire.
These FSL antennas are more difficult to construct when factoring in the optimal number of rods, and Litz wire turns for desired ~ 520 to 1750 KHz coverage. There are many factors involved that need to be considered before purchasing materials. Kevin Schanilec's FSL optimization paper is essential reading before undertaking construction.
Use cheap magnet wire at first to find out the turns needed, before going to the expensive Litz wire. That will still be just an approximation not an exact thing. Big Litz of course is wider and for a given coil width will have fewer turns than wire that is thinner. All that factors into the final coil inductance. As noted, almost all FSL coils require experimenting with the actual number of turns for the variable cap being used. Main thing is to have enough inductance to tune to the bottom of the MW band.
Longer ferrite rods are always preferable to shorter rods, given a choice. The current batch of Russian rods on eBay are all 400 permeability, which is pretty high. Desirable permeability for MW is 125. If the Jaycar rods are similar to 400, then fewer turns would be needed for a given inductance than for rods with lower permeability. If they are 125, more turns are required. So once again it's all about experimenting to figure out what the Jaycar rods do.
Gary discovered a formula for FSL sensitivity relating FSL coil diameter to FSL coil width with larger diameter and longer coil length giving greater sensitivity. Generally you want the largest diameter FSL your pocketbook can afford to buy ferrite rods for.
You mention Q, but generally Q is related to the size of the Litz wire, and if the variable capacitor has good Q. Generally coil Q sets the limit as generally coil Q is lower than cap Q, if the cap is decent quality. Variable caps with phenolic insulation are always lower Q than other types of insulation; the caps Gary now prefers have phenolic insulation but apparently the manufacturing process somehow makes these with a higher Q than previously.