How large would a database containing perfect knowledge of chess be?
Claude Shannon indicated that the State Space Complexity (number of legal positions reachable from the initial position in chess) has a lower bound of approximately $10^{43}$. If each entry consists of $20$ bytes, as indicated in your question. The total number of bytes for your database would have a lower bound of $2 * 10^{44}$ which is a very large number indeed.

I would suggest reading Section 2: General Considerations of Shannon's paper, Relevant quote:


  Another (equally impractical) method is to have a "dictionary" of all possible positions of
  the chess pieces. For each possible position there is an entry giving the correct move
  (either calculated by the above process or supplied by a chess master.) At the machine's
  turn to move it merely looks up the position and makes the indicated move. The number of
  possible positions, of the general order of $64! / 32!(8!)^2(2!)^6$
  , or roughly $10^{43}$, naturally
  makes such a design unfeasible.


References:


https://en.wikipedia.org/wiki/Game_complexity
Programming a Computer for Playing Chess - Shannon