By and large, large ORC files are better. HDFS has a sweetspot for files that are 1-10 times the block size. But 20GB should also be ok. There will be one map task for each block of the ORC file anyway. So the difference should be not big as long as your files are as big or bigger than a block.
Files significantly smaller than a block would be bad though.
If you create a very big file just keep an eye out for stripe sizes in the ORC file if you see any performance problems. I have sometimes seen very small stripes due to memory restrictions in the writer.
So if you want to aggregate a large amount of data as fast as possible having a single big file would be good. However having one 20GB ORC file also means you have loaded it with one task so the load will normally be too slow. You may want to have a couple reducers to increase load speed. Alternatively you can also use ALTER TABLE CONCATENATE to merge small ORC files together.
More details on how to influence the load can be found below.
http://www.slideshare.net/BenjaminLeonhardi/hive-loading-data
