You can easily add limits to your reports to keep them from showing too much data. From the report editor, click on Configuration -> Limits. Depending on the report type, what's shown in the limits will vary. For a list report, you'll see:
You'll need to choose a field to use as the limit. For example, if you want to limit to the top 10 rows by order total, you'll want to choose your order total field. Once you have that field selected, you can choose the number to limit to, whether you want to limit to the top or bottom N of rows, and whether or not you want to group all other rows into an 'Other' category.
For chart reports, the options will be similar, but you don't have to specify the measure for the limit since it's implicit within the chart. With stacked column or charts, you can also choose to limit the number of entries within the stack of each bar:
