The Flower Power theme is designed to be left aligned and exactly the size it is appearing on your blog. if you wish to make it larger there's only one way to do that is by purchasing an annually renewable CSS editing upgrade.
If you have CSS editing experience, you can get the paid custom CSS upgrade and renew it annually. It will allow you to stylize the appearance of themes you find here > Appearance > Themes, but it does not allow you to change the functionality, by editing the underlying template. Also note that CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
Since there is no official Staff support for the CSS upgrade, and precious lttle Volunteer support I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
Alternatively you can simply change to another theme. You can use the search utility and Feature Filters in your Dashboard > Appearance > Browse Themes > Feature Filters to locate themes of interest to you. (A-Z, Popular, Features)
Each theme has a thumbshot and a features summary description. You can browse and when you find one of interest you can click “Preview”. When you are ready to make your selection you can click “Activate”.