change boxcollider2D length during runtime

how can i change where my boxcollider 2D moves from point a to point b because in my game i have a line which bounces off walls and may move in odd directions and might collide with an object and i need this box collider 2D to stay on this line wherever it may go I've tried using the same position as the line but it docent make any sense I've also tried looking through unitys website to find help does any body know how to accomplish this thanks