Here are some of his minor league cards, plus an oddball.
From the 1988 San Bernadino Best team set:
Griffey 1988 Best.jpg
From the 1988 San Bernadino California League team set:
Griffey 1988 Cal.jpg
From the 1988 California League All-Star set:
Griffey 1988 Cal A-S.jpg
From the 1992 Best set:
Griffey 1992.jpg
And, from the 1990 All-Star Masks booklet set. The page is approximately 11.5" by 13.25".
Griffey 1990 mask.jpg