From Unofficial BOINC Wiki
The term Work Unit has a double meaning. It is technically a portion of data that a Project needs analyzed. It is copied into a varying number of Results which are then sent to Participant's Hosts for processing. Common sense tells us that we are sent a Work Unit not a Result and the term is often used that way.
The size and the content of a Work Unit is strictly Project dependent and the size may vary from small (200 to 500 Kilobytes) to very large (several Megabytes). The processing time will also vary on a Project dependent basis, with a currently known range of a few minutes to months.
Since the average size of the Work Units is known in advance, those who are to participate in a Project will know up front how large the Work Unit will be for that Project. More specifically, the Projects' Scheduling Server and Data Server will know the characteristics of the Participant's computer and will not send it work that cannot be completed by that computer.