To change the size or the resolution, just open the blog and go to layout of the blog, then go to the edit html link.
Go to the outer wrapper section, in the editable xml file.
In the outer wrapper you will be able to see some thing like this,
Just change the text in the xxx part with the resolution you want, for example say, width: 700px;
Similarly change the width parameter in the main wrapper and the sidebar wrapper, to change their widths as well.
Main wrapper is the one in which your text occurs.
Width of the Side bar wrapper determines the width of the sidebar.