at full length—
1. with the body stretched out and flat:
- He never walked again, but was drawn about lying at full length in a sort of bed-carriage.
2. describing smth. thoroughly and in detail:
- The purism of modern times would not allow me to give this story at full length.
at great length— = at full length 2:
- He went on at great length about his favorite hobby.
Note: The expression does not fully correlate in meaning with the phrase at length—
1. = at great length:
- He dwelt at length on the considerable opposition to the entry of Britain into the Common Market.
2. after a long time or pause:
- At length he spoke again, softly, as if he was debating with himself.