OK .... found it RRS rule 77 and appd G ....
for ISAF classes depending on length:
Overall length Minimum height Minimum space
between characters and from edge of sail
under 3.5 m 230 mm 45 mm
3.5 m – 8.5 m 300 mm 60 mm
type style 'Commercially available typefaces giving the same or better legibility than Helvetica are acceptable'.....
for non ISAF classes then per CA or national authority rules.
Our CA does not specify for numbers (only class insignia). RYA assigns numbers and requirements for national classes (not including Phantom). but has an RYA rule:
APPENDIX G IDENTIFICATION ON SAILS
G1 Identification, Specifications, Positioning
The requirements of rules G1.1, G1.2 and G1.3 apply to all British owned boats except that:
(a) under rule G1.1(a), class insignia need not be carried when the boat carries a sail number from one of the series specified in G2(a), (b), (c) or (d) below.
(b) under rule G1.1(c), a sail number of more than four digits may be carried.
(c) under rule G1.1(c), for boats other than ISAF classes, one of the authorities specified in G2 below shall allot an appropriate sail number.
(d) under rule G1.2(b), the height of a letter included as part of a sail number shall be the same as any numerals included in the sail number.
(e) under rule G1.3(c), the national letters may be placed in front of or above the sail numbers.
G1.2 is requirement for 300MM .. so there you go.
