Bar size and how many bars per line

By default, MuseScore follows standard rules of engraving and fits as many measures on a line as it can given you current size and spacing settings (eg, the staff and page size in Format / Page Settings, the various distances in Format / Style / Measure). Just as a word processor nonrally decides how many words to fit on a line. In the majorty of cases you don't need to mess with these choices. But if you have some reason to want to end a line earlier, just add
a line break
like you would
in a word processor.

If you want more measure to fit than otherwise would, then - again, just like a word processor - you'll need to changes you settings to make the music smaller or page bigger.

How do I select how many bars per line...?

You can use menu item: Format -> Add/Remove System Breaks
but be aware that if you specify, say, 4 bars per line (system), the 4 bars may contain way too many notes (e.g. a continuous run of 16th notes) to fit. In such a case, a new line (system) will be created for the excess.

...how do I manually more bars to change the size of them?

Bars change size depending on content. For example, a bar of 16th notes requires more space than a bar containing a single whole note (in 4/4). MuseScore spaces bars according to content.
Because of this (and other factors such as lyrics, fingerings, etc.) it is best to format a score after note entry.