You can right click a contact in your roster and select “Send a file”. If you open a chat window with a contact, then at the top, the 3d button will do the same. You can also just drag a file into a chat window and release it. It then should start a file transfer.
You can limit file size via default.properties. But you will have to produce your custom spark.jar and distribute to your users (you will have to do this for every new version of Spark). Spark default.properties guide (updated)