Once your source is uploaded to BigML you cannot reduce the size of your source. The size will remain as your original source file. However, you can take a portion of that source if you want to create a smaller dataset or any other of the BigML resources. For instance, say that you want to work with the Development mode where your tasks cannot be larger than 16 MB each, in this case you need to use only a portion of your source file.
BigML lets you decide the percentage of data to be used to perform any task either thru the BigML Dashboard and the API. If you are using the former to create a dataset, you can reduce your source by just moving the highlighted slider on the image below, which you can access from the source view, and clicking the Configure dataset icon:
This is the argument you need to set if you are using the BigML API.
Notice that when you have created a dataset you can also use just a sample of it to create other BigML resources. For more information, please take a look at the Sampling Datasets chapter in the BigML Dashboard documentation or read the subsection sampling of the documentation for developers.