I have that they were 29 up until 1999 and then went to a 28mm bar in the 1999 cobras
This is the little list i have compiled
2003-04 Ford Mustang Mach 1
Front Springs 600 lb unique linear rate (.75 in drop)
Stabilizer Bar 26.5 mm (1.04 in.) solid
Rear Springs 250 lb unique linear rate (.75 in drop)
Stabilizer Bar 23 mm solid (.905 in.) solid
2001 Ford Mustang Bullitt
Front Springs 600 lb Linear (.75 in drop)
Stabilizer Bar 26.5mm Tubular
Rear Springs 250 lb/in. Linear (.75 in drop)
Stabilizer Bar 21 mm Tubular
1999-2004 Ford Mustang GT
Front Springs 450 lb/in. linear
Stabilizer Bar 28-26.5 mm (1.04 in.) solid
Rear Springs 210 lb/in. linear
Stabilizer Bar 24-23 mm (.905 in.) tubular
(1999-04 Bars claimed to be different thru years)
1999-02 Ford Mustang Cobra
Front Springs 500 lb/in Linear
Stabilizer Bar 28mm
Rear Springs 470 lb/in. Linear
Stabilizer Bar 26mm
2003-04 Ford Mustang Cobra
Front Springs 600 lb/in. Linear
Stabilizer Bar 29mm
Rear Springs 600 lb/in. Linear
Stabilizer Bar 26mm
1996-1998 Ford Mustang Cobra
Front Springs 400-505 lb/in. Progressive
Stabilizer Bar 29mm
Rear Springs 165-265 lb/in. Progressive
Stabilizer Bar 27mm
1996-1998 Ford Mustang GT
Front Springs 400-505 lb/in. Progressive
Stabilizer Bar 30mm
Rear Springs 165-265 lb/in. Progressive
Stabilizer Bar 24mm
2000 Ford Mustang Cobra R
Front Springs 800 lb/in. (Coilover)
Stabilizer Bar 28mm
Rear Springs 750 lb/in. (Coilover)
Stabilizer Bar 26mm
87-93 5.0
Front sway bar 1.3"
rears sway bar 0.83"
79-82
Rear sway bar 0.55"
1983
rear sway bar 0.67"
1984 (early model change)
rear sway bar 0.83"
rear sway bar 0.79