Lloyd, If (big If) I am understanding you correctly, here are some possibilities.....

1)Controlling the length of the line..... Is that in printing or writing the music in? If it is printing, there is a check box in 'page setup' under 'options' which extends the last system.... If it's just on the screen, don't worry about it because it only adds measures as you add them.... As far as setting the number of measures on the printed page/line, you can't really adjust that except by setting your margins and font size... Again, this is in the page setup.

2) When you are printing two lines, is that ie, piano, rh/lh, or two vocal lines or something like that? The way to make the measures line up is to make sure that you have the same number of beats in each bar using notes and rests..... sometimes you have to play with the configuration of the beats a bit.... Also, check you use of tie's and slurs. That sometimes has an affect on the spacing, but mostly it's having the right number of beats in each bar.

Hope this helps.....

musicman