A Datablock is a unit of space allocated to Table(not shared between tables) Datablocksize gives the maximum permitted size of the block that will be allocated for the table.
Freespace percent is the percentage of cylinder space to leave free during load operations. (To accomodate future insertions or row updations in the same cylinder)
There are lots of things to keep in mind w.r.t Datablocksize. some of them are... - If the size is large then we can have multiple rows in the same block and it would be efficient when it comes to retrieval as multiple records can be obtained from a single I/O. - If size is less then its possible to hold these blocks in memory and process them faster
Please refer the technical documentation available at the Teradata site for more/specific details about these topics.