I think I would disagree with my learned friend from Hampshire.
The Laws of the Game (2013)clearly state 4.3.(a) "studs of players' boots must conform with the IRB specifications (Regulation 12)." which says 13mm at the base and 10mm at the top.
a quick read of the regulation will indicate that is not the case.
"Stud/cleat length shall be no greater than 21 mm
(see Law 4).
Studs/cleats complying with the design and
dimensions shown in Figure 1 should give
satisfactory performance.
The shape and dimensions of other stud/cleat
designs should be such that they present a no
greater risk of injury to another player than the
stud/cleat shown in figure 1. Tests A and B can be
used to assess comparative performance.
The plan view cross-sectional contact area of the
stud/cleat shown in Figure 1 at a plane 2 mm below
the tip is 78 mm. Other studs/cleats having the
same or greater contact area might be expected to
give satisfactory performance dependent on
minimum stud/cleat width in any direction.
All edges of the studs/cleats should be finished
smooth and rounded to a radius of not less than
1mm. "
The only stipulated dimension is length 21 mm. As you can see there is reference to 'other stud/cleat design' as long as they 'present no greater risk of injury.
Figure 1 is a representation of a design that 'should give
satisfactory performance'
Regulation 12 is a 'GENERAL DESIGN GUIDANCE
GUIDANCE FOR SHOE AND STUD/CLEAT MANUFACTURERS '
It is not a directive to Referees.