4/0 F9-F10, string will be from 2-1 inch
3/0 F8-E9 string will be from 3 to 2 inch
2/0 F7-E8 string will be from 4-3 inch
1/0 for F6-E7
1 for F5-E6
2
3 for F4-E5
4
5 for C4- E4

7 to 21 for C2 (based off this video, he calls it .48 gauge string???)) ¹⁾

3“ x 15 3/4”
Intended range from F7 to F10
Each string is 4 inches Long, with bridges to change length down to 1 inch as needed.
Major notes end pin holes are 1/2 inch apart with 1/2 inch gap from the top, 4 1/2 inch gap on the bottom.
Accidentals end pin holes are 1/4“ off then 1/2” apart. 3/4“ gap on top, and 5 1/2 inch gap on bottom.
Pegs go along the center starting at 4 1/4 inches from the top, pegs are every 1/4 inch.
The major notes are staggered 4mm from center, with the accidentals staggered 4mm on the opposite side.
Peg holes are drilled as 3/16th holes that go 1 inch deep, for using zither pins.
End pegs are 3mm from the edge of the wood, and drilled with 1/16 inch drill bit at 1/2 inch deep
End pegs are #17 1” long stainless steel nails, with 1/2 inch left exposed.
The string knot nail holes are 1/4“ down side walls at same locations as the end pins.
the string knot nails are #18 3/4” (or 1.2 inch) stainless steel nails with 3mm left exposed.

If this prototype works I plan on making 3 copies, and with some luck the copies will also be able to be lowered to match the structure but with wider strings to allow for lower pitched tuning

Eb 10 (19876 hertz)

It seems most psaltery's choose string size by the octave, but it also seems many have the lowest octave divided into 2 sets to allow the lowest notes to be tuned down enough
4/0 string for the upper register octave E9-F10
3/0 for the middle register octave E8-Eb9
2/0 for the lowest register octave F7-Eb8
1/0 for F6-E7
1 for F5-E6
3 for F4-E5
5 for C4- E4

see music wire