Xrepeat
xrepeat and yrepeat are members of the sprite and wall structs, where they control sizes. The term "repeat" connotes "how much distance should there be before another copy of the tile were to be drawn".
The range of values is 0 to 255, inclusive.
Sprite
xrepeat and yrepeat are the x and y scale of the sprite, respectively. These are the same values that are set by the sizeat command. Thus:
setactor[THISACTOR].xrepeat 40 setactor[THISACTOR].yrepeat 40
is equivalent to
sizeat 40 40
Setting a sprite's xrepeat to 0 is equivalent to calling the killit command from that actor. Change the actor's statnum as well if they are not STAT_ACTOR for this to take effect.
Wall
In the wall struct, xrepeat and yrepeat are the size\stretch of the texture used (if any).