You may always use user_segmetns instead of dba_segments, and user_Segments is a VIEW much more frequently availble to users.
As for the size of the WMSERVICE table, It highly depends on the size of the pipeline. Save the sample input pipeline to the XML-file, check it size, double it, and it will be sample size of the BLOB.
Then consider that:
having blobs larger than 4000 bytes forces out of line storage, so the space will be alocated in 4k chunks.
If the bobs are smaller than 4000 bytes, they will be stored inline, and they will take exactly the space, the blob size is.
So, considering, that 90% will fit in 4kb and 10% will fit in 8kb, it looks like you need about 3GB + indexes.
To efficentli store 90 days of history, you may consider table partitioning, this is what we have done, but we have ~1.2M entries a month, and the table takes 5-8GB per month.