How can i shorten the last line so that the bars are the normal size?

append a horizontal Frame and adjust ist size

You can also try increasing the "Last system fill threshold" in Style / General / Page to 100%. This will prevent MuseScore stretching the last system to the right margin. However, it means the measures will be some minimum size that will most likely be *smaller* than on other systems, because the other systems *are* being stretched to the right margin. That is, most systems have measures that are bigger than necessary because of the normal stretching that happens, and each system is stretched by a different amount. So it ends up being a subjective matter to decide how much stretch you want applied to the last system - which systems' measure would you like it to come closest to matching, since they are all different?

Meaning, in the end, appending a horizontal frame and deciding for yourself how big you want it is usually the way to go.