How to determine BGA land pad diameter for given ball diameter?
Well a good place to start for questions like these would be the IPC spec PC-7351B not free but it has all the information you'd need about land patterns for most any part.  

Second I like to use the free version of the pcblibraries tool because it automatically generates IPC compliant footprints from your input data, and can then output or create them in your CAD program.

Finally here's a link from mentor describing more than you'd ever want to know about BGA pad size.   For instance it shows this IPC table:



With your ball diameter being 0.25mm, that looks like a land pattern of 0.20mm.