The size of the banner space is dependent on what layout you've selected, and what width you've selected for your layout. Within the Appearance Editor of your style there will be a recommended banner width, based on that individual style. You can view this from the Banner & Navigation tab, under the Banner Image (URL) field.
Alternately, you can measure your site to determine the size for your banner image. To measure your site, we recommend using the editing tool you'll be using to make your banner to measure the size of the layout. You can get a snapshot of your screen in windows using the Print Screen key, which you can then paste into your image editing program using the Paste function.
After adding your banner, you can adjust the padding/margins for the Header and/or Banner Image (if added) elements to adjust the position.
Note: We have a page here that lists free and paid options for image editing/creating.