This article is from the Fencing FAQ, by Morgan Burke with numerous contributions by others.
Foil guards vary mostly in diameter, being between 9.5 and 12 cm
across. The largest guards (eg. Negrini) may fail the weapon
guage check if they are dented or misshapen.
Epee guards are almost always the maximum diameter (13.5 cm) for
best protection, although they can vary considerably in profile
shape, depth (3 - 5.5 cm), weight, and eccentricity (up to 3.5 cm
off of center).
Sabre guards come in left- and right-handed versions (the outside
of the guard being larger). Competition guards may include
attachments for a capteur sensor. If not done by the
manufacturer, sabre fencers may wish to insulate the edges of the
guard (and the pommel) to prevent it from shorting to their cuff.