The sidebar width is determined by this rule:
.yui-t2 .yui-b, .yui-t4 .yui-b {
width: 15.6em;
}
If you increase the width, though, you start to overlap the centre column. So you have to make compensating changes to this rule:
#yui-main, .yui-g .yui-u .yui-g {
width: 100%;
}
For example, you could try:
.yui-t2 .yui-b {
width: 17.5em;
}
#yui-main {
width: 98.25%;
}
It would be nice to put your other questions in separate threads. Keeping to one topic per thread helps other users when they search for answers.
You should also be aware that the only recommended way of making changes to themes (unless you have complete control over the theme’s code) is via a Child Theme.
HTH
PAE