There are no rules governing the length of dance figures in general; Each movement simply takes as long as it does, based on the total number of steps taken and the timing used. Some figures will begin on beat 1 of a measure while others will not, but this is not necessarily wrong. In fact, it contributes to musicality by adding variety and interest.
In Foxtrot as well as most Swing-related dances, including EC & WC Swing, Lindy and Jive, most basic patterns are built on a combination of 6- and 8-count rhythmic structures. The 8-count patterns fall neatly into the minor 2-bar phrases, but the 6-count patterns do not. Instead, they alternate between starting on beat 1 of one measure and beat 3 of the following, so that two 6-count patterns take up 3 measures of music.
Regards,
Jonathan