Extend last bar length?

There's a few ways, depending on how what you want. the reason it's small is because it's on a line by itself. So one way to address the problem is to make that not be true any more. What *I'd* do is select all, reduce stretch (see Layout menu), and hope that in itself was enough to push the bar up to the previous system. I might use manual line breaks (see Breaks & Spacers palette) to force it to four bars per line. You could also play with the overall scale settings in Layout->Page Settings - I suspect you'll find the default used in the PIano template to be rather small when printed. this would probably results in some lines getting *fewer* measures, which again would give that last measure company.

But if you're OK with everything else and just want to stretch out that measure, the way to do it is to append a horzontal frame (see Create->Measures), and then adjust it's size to take up the space that the "missing" measures on that line would have taken. The single measure there should then stretch itself out.

Try Style > General Style > Page and set last system fill threshold to 0% and see if that helps. I just tried it and it worked you your file! :-)

As an aside, click on Breaks & Spacers and drop them at the end of each line to control where the breaks are. I deleted that thing ahead of your first line too and everything looked lined up.