King Sigismund II Augustus as well as his successors attempted to create a proper naval fleet, but their attempts met with repeated failures, due to lack of funds in the royal treasury --due to the opposition of both Polish nobility and Gdańsk city to the idea of a royal fleet. Once built however, the Commonwealth Navy managed to defeat the Swedes at Oliwa (1627), so securing for themselves a permanent access to the Baltic Sea. The Polish-Lithuanian fleet was entirely destroyed in 1637 by Denmark and no funds were found to rebuild it anymore.
Sources: Wikipedia(en):Polish Navy | Flags of The World (FOTW) website:Poland - Historical flags | Historical Flags of Our Ancestors website:The Flags of the Poles and Poland - Part 1, 900 to 1795
Sizes: ensign W12 x L22 mm, mast flags W8 x L12 & W8 x L15 mm, jack W6 x L9 mm, pennant W4 x L48 mm.